I learned after a couple of years of quilting that I should block my finished quilts. This is not something I had ever seen in books when I was learning to quilt. Perhaps that is because it is not as necessary in large bed sized quilts or quilts that are quilted on a frame. Quilts such as wall hangings, crib quilts and lap quilts show up any warping more because of their small size. I machine quilt on my home sewing machine, and that can further warp the quilt because of all the "acrobatics" involved in getting all that bulk through the narrow throat of a sewing machine. In any case, I love the look of freshly blocked quilts, so I always take this step in finishing them.
First of all, I throw my finished quilt in the washing machine after machine quilting it. I do this because I use spray basting to baste the layers together before quilting. I also use chalk or other means of temporary marking to mark quilting lines and I want all of this removed. I also like the slightly puckered "antique" look washing gives my quilts. It slightly shrinks everything and tightens it up. If anything is going to come unsewn, it will most likely happen during washing, so I can repair it. I like to wash and block my quilt after sewing the binding on, but before turning it and sewing it to the back. In this way, it tends to lie down flatter as opposed to getting those little diagonal wrinkles caused by shrinking after washing. I do a gentle wash in cold or warm water, using only a fraction of the usual amount of detergent. I let it go through the final spin to remove any excess water.
In preparation for blocking, I put a white vinyl tablecloth (the type with a flannel backing) on a thick rug or carpet. I put the vinyl side down. You can also make a blocking mat of foam core insulation covered with a white sheet. The idea is to have a surface that will holds pins.
Lay the wet (but not dripping) quilt on this surface and begin by squaring one corner. Pin every few inches and gently pull the edges into alignment if you need to. The quilt is quite malleable while wet. Note that the binding has NOT been turned and sewn at this point.
After squaring up the first corner, pin the edge leading up the the opposite corner straight. It should be taut. I did the top two corners, but you can start anywhere. Now square this corner in the same way.
Start pinning the sides of the quilt down, making them straight and measuring across the quilt as you go to be sure the measurement stays the same. (Measure from top to bottom also to be sure the length of the quilt stays the same.) When you get to them, square the remaining two corners as you did the first.
You can also block old quilts or previously finished quilts. If you don't want to go through the washing machine step, you can spritz them with water until they are wet enough to be malleable. Yes, you can block them after the binding is sewn on as well as before. If you have never blocked a quilt before, I think you will really like how nice the quilt looks after this step. Give it a try!