Bonnie recommends using old phone book pages for our paper foundations. But I worry about the ink transferring when they're ironed. So I'm going to cut my squares from pattern-tracing tissue paper. I've got a whole roll of the stuff. (I also do garment sewing.) It's thin and is easy to tear away after stitching.
In my first paper piecing project (which was about eight years ago), I made the mistake of using 20 lb. printer paper. Well, they say there's no teacher like experience, and it's true. I'll never do that again. That stuff was not only hard to rip out, little bits of it were left inside the stitches and I had to pull them out with tweezers. It turned me off from paper piecing for a long time and made what was supposed to have been a fun, quick project into tedious drudgery.
When I cut out my paper foundations, I plan to use my quilting rulers and an old rotary cutter with a dull blade. It's good enough for paper and much quicker than scissors!