There is NOTHING wrong with the way you sew quilts. Completion to me means a lot, so I get pleasure out of the completed project. However, I have never used a "bought" pattern. I pick a block that I like for the fabric selections, so it might be something like a 8"snowball block. Then I figure how big I want the quilt... say a full size. That's about 80 x 90. Then I divide the size of block (8") into the 80" (L) and you get 10 blocks. So 10 - 8" blocks will be the length. Then I divide the size of block (8") into 90" 9(w) and I get 11. So the width will be 11 - 8" blocks. So its 10 blocks x 11 blocks. Since there is a 1/4" seam the cut block will be 8 1/2 square for all squares. Finished block will be 8" square. For sashing you would have to add that amount to the block and/or add sashing to the top and bottom amount to the block. This will be something simple for you to handle and I am sure you can do it. The small squares that fit into each snowball corner are 2 1/2 each (4 per block - 1 for each corner).
Have fun experimenting.