I would just add the sashes to the squares if you need to make the sqaures all the same size. I know you can do it! When I'm facing a lack in confidence I simply call upon my quilting Angel and ask her to guide my hand when cutting material or ask her to help me understand a poorly written set of instructions. I also ask her to calm my mind so my full attention is on what I'm doing and not other issues that don't really need my attention when I'm sewing.
I hope this works for you!