I became really picky about things like always straightening the grain on my fabric before I cut so I don't get wonky strips.
I cut on the outside of the line on my ruler so every piece is just a sliver bigger than its supposed to be and then I cut the block to size at the end. And I have rulers in all the block sizes I make so the I can square them up exactly before I begin sewing them together.
I also have a hard time conceptualising so I spend a lot of time planning on paper and just staring at things on my design wall before I do any actual sewing.
Watson