I've made two t-shirt quilts -- one for my chapter to keep in the tv room and one for my husband. They're relatively easy, just take some time assembling.
1. Decide what size you want your quilt to be, and whether you want strips of fabric dividing the squares of t-shirt. Measure and cut the t-shirts (adding 1/2" or 1" extra on all sides for seam allowance (1/4" seam if you added 1/2"; 1/2" seam if you added 1")). The easest way to cut is to first measure a cardboard template, then trace the template on the t-shirt, then cut using a rotary cutter, ruler and mat.
2. Iron stabilizer on the backs of the cut-out t-shirt squares. You need to do this so they don't stretch.
3. Arrange t-shirts on the floor in the pattern you want for the quilt. Pin together, two at a time, and sew until you have a row completed. It's easiest to sew by rows across when you're sewing the entire top together.
4. Once you have the top sewn together, lay down your backing (a flat sheet works well), batting and quilt top in that order, just as they will look when the quilt is finished. pin together on the edges and through the middle. Sew together with a basting stitch first to keep things from sliding and then fold over and pin the backing 1/2" over the top to create your binding. Sew together. Tie quilt every 6" or so.
Here's the one I did for my husband using a king-size sheet for backing:
