I'm in the process of making my first bargello too and the pattern I'm using calls for different widths of strip sets. It's only the wider ones that are a bit tricky and I'm finding I have to square them up a bit after every few cuts. They're well pressed, all seams going the same way and I kind of have to splay them out flat with my right hand/forearm while I get the ruler in place with my left hand. A bit ungainly but it seems to be working! Getting someone to help would be easier!