My mother told me it is getting oils from your hair, that make it glide more easily into your fabric. She was a professional seamstress sewing for models.
Place the quilt flat on the floor or a table. Get someone to help you if you need it. For the right and left (vertical) borders, measure across the middle of the quilt from top to bottom. Measure more than once, and make sure the quilt is flat where you're measuring. Cut both borders to that measurement. Sew them on. Then measure at the middle from right to left in the same way. Cut two borders to this length and sew them onto the top and bottom edges of the quilt.
Mitered borders need to be handled differently.
I bought a full-size roll of Warm and Natural quilt batting, but any kind that will stay together will work. I sewed a hem in the top and bottom and asked DH to find me some rods that could go in them. He came up with some batten-type strips of wood from the garage that exactly fit into some cup hooks I installed on each side of the top. It stays rolled up in a corner when not in use. Whenever I need my design wall, I unroll it, put the top strip of wood into the cup hooks and I'm good to go! (I need to use pins if I'm going to leave blocks or sections of the quilt on it for more than a few minutes.)
If you need to cut a bigger piece of fabric than your biggest ruler, you can do the same thing. Just add another ruler, so you have the right size where you cut.
After cutting, check your cut fabric for a V indicating it wasn't cut at right angles to the fold. I check every cut, but you should be especially careful after cutting several strips, especially if they're narrow ones.
I have a blue cutting mat. Sometimes if I'm cutting a dark fabric, especially a blue close to the color of the mat, it's hard to see the edge of the fold. In that case, I place the fabric wrong side up, so the lighter color will provide contrast. This won't work with batik, because it looks the same on both sides. I use my gray travel cutting mat for blue batiks.