Cutting Welting Strips
Use a ruler to draw 1-1/2" strips. It's OK to use a sharpie to draw the lines.
The strips can be on the bias, cut horizontal, or cut slightly on the diagonal. Upholsters call the last a "60 inch bias", made by laying a 60" ruler across the 54" width of fabric. I'm using this method as it's just enough bias to help the piping on curves, but doesn't take up as much fabric as a 90 degree bias would.
Next I cut strips using my Sailrite Hot Knife and tempered glass sheet. Here's a video of strips being cut with the hot knife
Assembling Welting
For the welting I bought 5/32" polypropylene outdoor welting cord from Sailrite.I've also purchased 1/4" basting tape from Sailrite. The basting tape is applied just inside one edge of the welting. Then the tape is peeled back to reveal the double-sided sticky tape. The piping is laid inside and the edge folded over, and finger-pressed to close. This eliminates the need to baste by machine and eliminates any issues with extra welt stitching showing when the cushions pieces are sewn together.
Update: after doing a few cushions I decided it's just as easy to baste by machine, so I have lots of the basting tape left over.
