Squaring up a quilt really isn't that hard, but it is a step that slows you down when all you want to do is get that binding on and see what you quilt is going to be. It always helps to stay as square as possible along the way. Check each block as you go, and fudge seams if necessary when putting tops together. If you are using borders there is also a way to help bring a skewed quilt into square or ensure it stays square, but that's another tutorial!
I've always done my best to stay as square as possible during construction, but the evolution of my design aesthetic now sees the majority of my quilts finished without borders and not necessarily square blocks. That same evolution has led me to the following technique. See if it works for you.
You don't need any special tools to do this. Your sewing machine, thread, a large table top, self-healing cutting mat, rotary cutter, and a ruler at least 12 inches square.
Before you get started decide on how much extra batting, if at all, you want your binding to contain. I cut a quarter inch from the edge of my quilt top. My binding is then attached flush with the quilt top's edge with a quarter inch seam allowance. Folded over I can then hand sew the binding and it perfectly covers the seam from attaching the binding. If you want a less substantial binding then you could cut closer to the top's edge and use a smaller binding.
Here's how I do it.
1. Set your machine to a zig zag stitch that is small in width but long in stitch length. Stitch around the entire perimeter of your quilt, through all three layers. This essentially turns your quilt a solid piece of fabric, reducing the potential for movement when you attach your binding.
Ensure you stay as close to the edge as possible. If you have a walking foot, use it. I have an even feed foot on my machine and that's what I use. Oh, and take it from me, ensure that you have a throat plate in your machine that can accommodate a zig zag stitch! I nearly lost an eye when my needle broke. This stitch will be entirely hidden by your binding.
2. With your quilt supported on the table (if it hangs over the weight may pull you out of square as you cut) get ready to cut. The whole quilt doesn't have to lay flat, so don't worry about having a giant table. As long as your cutting edge and about 12-20 inches to the side of that edge stays flat.
3. Start in one corner. Line up the corner of the quilt top with the corner of the ruler, a quarter inch in from the edge. This is where the actual process of squaring begins. From the corner you will want to keep the quarter inch markings in line with your quilt top's edge. To do this you may need to pull the quilt into square. Hold the ruler down firmly but not so hard that you can't pull the fabric with your cutting hand. I find that my thumb holds the ruler more firmly, and thus holds the quilt in place, and that I can move the fabric near the top of my ruler. (see picture below). Pull the quilt, holding on to the batting and backing fabric until it is in line with the quarter in marks. You may have to pull the inside of the quilt in, rather than the edge out. Just go slow and pull as necessary. When you have it lined up with your ruler, cut it for the length of the ruler only.
This is what it looks like.
As you finish one cut, move the ruler up the edge of the quilt (keeping the quilt on your cutting mat). Constantly realign the quilt top's edge with that quarter inch mark. Pull the fabric as necessary to bring the quilt top's edge into alignment. Always hold the ruler firmly as you cut.
Continue around the edge of the quilt until you get to the next corner.
4. When you get to the corners ensure your quilt top edge lines up in the same way as the edge you are currently cutting. In other words, you should be cutting your vertical edge, As you approach the corner - at least 8 inches out - line up your ruler on the current corner as you did the first one. Square your top, or horizontal, edge by pulling into alignment prior to finishing the vertical cuts. Cut both sides of the corner at the same time.