Like the experts on this board always say : accuracy is everything in quilting. It starts with accurate cutting : measure twice, cut once. Then accurate 1/4 inch seams: practice and measure after you practice. Press your seams pretty much as you go. Measure each block as you finish it, and square it up if you need to.
Tedious? Yes, but if you take your time and practice these things with your first few quilts, it'll become second nature to you.
And, remember, have FUN!