I first got the idea of quilting in sections from Barbara Bonesteel's Lap Quilting. But Marti Mitchell's book helped me by showing several ways to do it, and I have used her ideas since.
I think that usually when it comes out a different size its partly due to not sewing truly accurate seams. I do sometimes have that problem, and have sometimes cut larger, especially when dealing with triangles. I'm still working on accuracy, and I think it comes from careful cutting and being sure that your 1/4 seam is true. Try your needle position out using some graph paper (for example) to find out where your true 1/4 position is. Sometimes I move the needle to the right 1 notch to sew just the teensiest bit less than 1/4 and I can usually get the patches to match. Be sure to match seams carefully, too. I think its something all of us beginners struggle with until we get it down. I have only been quilting for about 4 years, but I really enjoy trying new things. Right now I'm working on purses as a break from a large project.