If you don't feel like reading all of the following stuff, just skip to the last paragraph.
When I first developed my method of paper piecing I used regular copy paper because I wanted to eliminate the need to purchase special papers. However, in the early years I experimented with all different kinds of paper and discovered 17lb vellum. It was perfect for the process. And when I taught live classes I supplied the vellum patterns as part of the class materials.
Then came virtual classes and you all needed to print your own patterns. Copy paper is easily available and almost everyone has copy paper. It works, but there's more pattern prep to do because copy paper isn't smooth and isn't translucent. For most of my patterns, copy paper is just fine because the patterns don't have lots of segments. For instance, "Not Your Daddy's Log Cabin" blocks have twenty-two segments and regular copy paper doesn't quite cut it. It's usable, but not ideal. Thus, the need for vellum.
Please be aware that any vellum heavier than 17lb is too thick and heavy. It's more difficult to work with and is more fragile
So the bottom line is this...get the vellum. It goes through the printer the same as copy paper. You'll find streamlines the paper-piecing process because it's smooth and translucent, thus eliminating time. Set your printer to "Best" quality printing and give the ink a minute to dry before you start handling the patterns.
But What if I Can't Get Vellum?
Not to worry! Right now, 17# vellum is not easy to find; however, these alternatives are much easier to purchase, and in some cases work even better than the vellum:
June Taylor "Perfect Piecing" Transparent, Blank Quilt Block Foundation Sheets
Simple Foundations Translucent Vellum Paper
Staedtler 100% Rag Vellum Pad 8-1/2" x 11
Pellon 806 Stitch N Tear Stabilizer - Cut into 8-1/2" x 11" sheets