If your top is more than 40" in either direction, your back will have to be seamed together unless you purchased fabric that is 90" wide. I sew the seam that takes the least work, usually right down the middle. Draw yourself a diagram of the size of your quilt. Assume 40" wide of usable fabric and see how long two pieces should be to make a back that is 4" wider and 4" longer than your top.
I will give you a hint. You can usually look at the measurement for your last border strips. If they were mitered, that figure has already factored in 4" extra and those are the figures you can work with. If the last borders were butted, just add 4". (See Measuring for Borders.)
Press your back.
You need a completely flat surface to batt your quilt. Two library tables pushed together is ideal.
When I was young, I worked on the floor. I prefer not to do that anymore, but sometimes it is the only way. It is better if you are working on bare floor, rather than carpet. Carpet gives and it will be hard to get your back taut and smooth. Indoor outdoor carpet usually has less padding under it and will be less problematical.
It would seem to go without saying, but be sure that the surface you use is CLEAN before you start.
Tip: Some people find that pressing the back with spray sizing or starch helps to batt without wrinkles and makes the back slide easier if you are machine quilting.
If you are working on a table, you can use binder clips to clamp the backing to the table. Let the top hang off about 2" at the top end, so the clamp is fully engaging the fabric. This also means you can line up the edge of the quilt along the top of the table. You should make an effort to have your back on grain with your top. Using the lengthwise grain of the fabric on the length of the quilt helps prevent wrinkles in the back. Having a straight line like a table top helps to keep all the pieces aligned.
Note: I have only one library table at home. Therefore, I have to baste in sections. If your quilt is larger than the table, be sure the top and one side are aligned with the edges, clamp around and baste what is on the surface. Unclamp everything and readjust the quilt so that unbasted areas are now on top of the table. fold back the top and batting and smooth the back into place. Fold the batting forward, then the top of the quilt and smooth away from the previously basted area. You want to be sure you are NOT creating a bubble where the first basting stopped. Repeat this procedure until the entire quilt is basted.
Here's a brief video showing how to move the quilt forward to the second section.
If the table edge will not take clamps (most dining room tables will not because they have a skirt) or you are working on the floor, you can use masking tape to secure the back to the work surface.
Draft someone to help you with this part if you possibly can. It is much easier to have people clamping or taping opposite ends of the fabric at the same time. You want the back to be smooth and taut but NOT stretched. If you stretch it tightly, it will relax when unclamped and make wrinkles.
Don't be tense about this. It is not hard to do. Tape about every 4" along all the sides. You can clamp a little further apart because they hold harder.
Now spread out your batting. It will never be the right size, so have it match the top and one side and let the excess hang off the bottom and the other side. Aside from giving you a clear idea of where the edges of your back are, it will give you the biggest possible batting scraps.
Smooth the batting from the center to the edges, being sure to get out all the wrinkles and bubbles. It is recommended that you unroll the batt the night before and spread it out to relax the folds, but that is not always possible. At my house, the cats would tear it into little pieces.
Tip: If you cannot unroll the batting and let the wrinkles relax overnight, throw it in your dryer on low heat for about 10 minutes.
Once the batt is as smooth as you can make it, add your quilt top. You want to line the top up approximately 2" from the top and side that you matched the batting to. The exact amount of this is not as important as keeping the two edges parallel. If you have let the back and batting overlap the table, you will line the top itself up with the edges of the table.
If you get those two edges in the right place, the rest of it will be right even though you cannot see all the edges. Smooth the quilt from the center to be sure all the bubbles are out of it. This is another place where it is really convenient to have a partner to help you center the quilt. Even husbands are qualified for this job!
Once you have all the layers smoothed out, you may trim off the batting. Keep the same 2" around the bottom and second side that you left on the top and first side. Do not throw this batting away. It can be used for other projects.
Note: The rest of the basting directions are given for hand quilting. To baste for machine work, just use #1 safety pins, spacing them no more than 3" apart.
Basting: Step 1
Next, you will start basting the three layers together. It is important that you baste thoroughly, so the layers will stay together without shifting while you are quilting . Thread your needle with a really long piece of thread, using any old thread you have. The only thread I would avoid is bright red or navy. The color could possibly rub off on light backgrounds.
Tip: I do not knot my thread, but you may if you like. I like to leave about a 4" tail. That way, when I am ready to pull out the basting thread, I can pull it from anywhere without trying to find the knot end. For the first two steps, you cannot knot it anyway.
Begin in the middle, leaving a really long tail so you can sew to the other end. Take long stitches to one end. Come back and rethread the needle with the long tail you left and stitch to the other end. Repeat this for the other crossbar.
If you were doing a really large quilt, you might want to stabilize the layers with a dozen safety pins, since you might have to crawl to the center.
Basting: Step 2
Next make a cross through the middle of the quilt. Then sew rows up and down, about 3" apart. The rule of thumb is that you should not be able to lay your hand down on the quilt without touching basting on both sides. The same rule applies to your quilting lines. If you know where you plan to do your quilting stitches, do NOT baste in the same place.
Note: These lovely diagrams have nice straight lines. If yours are not so perfect, it is no big deal. The object is to attach the layers, and wiggly lines do that just as well.
Basting: Step 3
Do not skimp while basting. Good basting makes the quilting so much easier. Finish up by bringing the excess back and batting around the front, fold them against the edge and safety pin down. This keeps the batting from getting in the way and prevents YOU from stitching the excess to the back of the quilt! It provides a kind of handle on the edges so you do not fray, stretch or soil the real edge of your quilt top.
You will undo these edges and baste the three layers together when you have finished quilting. After the edges are basted, you will use your rotary cutter and ruler to square the outside of your quilt.
To deal with quilting a big quilt on a home machine, Hari Walner uses a method called Quilting in Thirds. Lay out the quilt sandwich as always, but baste only the center section of the three layers together.
Fold back the top only on the left side and cut away the outside third of the BATT ONLY. Before moving it, mark the top and bottom on both sides of the cut line - A/A, B/B. Set that piece of batting aside. The letters will show you precisely where to put the piece back. Your cutting line does not have to be perfectly neat since it will fit precisely against the other piece.
Once the batting has been removed, roll up the top and back. Pin them with large safety pins so they do not flop around or get sewn to the back by accident.
Repeat the process on the other side:
You now have a long skinny quilt. It should fit under the head of your machine with no trouble. Quilt to within 2-3" of where you cut the batting. Some lines of quilting will be stopped in the middle of a line. End the stitching so it will not ravel loose but you will be going back and sewing over those places.
When you have completed the middle, lay the quilt out again. You can choose either side to do first. Unroll the top and back. Smooth the back out against the table and tape the loose edge. (The rest of the quilt MUST be sitting on the flat surface you are using or the weight will pull against the part you are unrolling.)
Put the batting back in place, matching the cut edges and the lettering. Using a long basting needle, make a big XXXXX row of stitches to attach the two pieces. These do not have to be tiny stitches that are close together. You can do it in a few minutes. You just want the two edges to stay in place as you pin baste the layers back together.
Unroll the top over the batting and pin securely in place.
Be sure that you quilt across the join as the first part of your quilting.
The bulk of the quilt is now to the outside of the machine and you are dealing only with this third.
When you repeat all the steps for the last third, you have to rotate the quilt so the bulk is still to the outside. That means you are working on the quilt upside down compared to the way you were sewing before. If you have directional motifs in your quilt design, be aware of this shift in direction.
The borders should be done last and can be done with the bulk to the left (or outside) of the machine during the whole process.
The only tricky part of this is making sure that you spread out the entire quilt when you are adding back the missing batting. It is tempting to take a short cut and unroll just what you need but that can cause a ripple on the bottom.