My twill tape arrived in the mail Saturday (all 72 yards of it! The best thing I could find at the time ...) It sounds plasticy, but its actually cotton so it adheres well.
That evening I got straight to work! After finishing my last greek key project, I realized that there must be some choice to fold the tape so you can do it consistently, without cutting each piece individually. At first I thought it would just give weird edges, but by folding it twice for one corner you are able to get square edges and 45 angles for the folds. Sometimes you dont get everything on the first (or second) try! :)
Heres how I did it for my 20"x20" pillow sham. First I made a sham using this method. I ironed it flat and laid out a painters tape guide line as shown in the diagram below (left image).
I created a border around the pillow using painters tape. I inset it a little and it ended up at about 1" from the edge of the sham. The keys were 3.5"x3.5", and I made the inner painters tape 1.25" thick (the distance of two of my 5/8" twill tape).
You can start anywhere, but you want to start the ribbon with a straight edge (slightly inset from where the tape will join it at the end, so that it gets covered) and then head either counter clockwise or clockwise. I found counter clockwise to be easiest for me.