I tried learning how to rotary cut when I first started and I couldn't cut straight. I'm not sure if I was even doing it right. So for the next 3-4 years I cut everything with scissors. Picked up a rotary cutter again and just cut out the strips for the binding and the shorter sashing. Had a little more success this time around, but it does require a bit of practice. That's why I only cut out smaller things like binding strips and such. I went on YouTube and watched videos on how to do it before jumping right into it....when I first did it I didn't have a lap top of my own and our family had dial up, which wasn't even realistic for uploading a short video. Actually seeing someone doing it really helped.
As far as sewing the pieces go, I mark my seam allowances on the piece that's going to be facing up when I go to sew and take it kind of slow.