I am not claiming to know anything but found when I started, the hardest thing for me was using the ruler and rotary cutter to get my strips, I had the usual V and still do sometimes in the middle of the fabric, or my strips would be off thus my blocks wouldn't be right. Still haven't mastered the accuracy of my blocks but I am getting closer. And what I found to be true and helps the most, first of all is practice. Second is laying it on top not beside. You can always cut down if its just a shade to large but can never add to. Hope this helps you but mostly it will come in time. Practice practice. Oh and that strip or even sand paper glued to your ruler will also help with the slippage. Good luck
Rita