If you are making a QAYG I would cut the square for the back out of good quality fabric, like using on the front. Lay batting on top I like to use different size strips that way I don't feel the need to match up lines. When batting is covered with strips, flip over and trim to block size.

If you are not doing QAYG, I used some of the deli papers to make a string quilt recently. I got these papers a long time ago and almost threw them out. I bought them for wrapping tamales in when I stemmed them. I only made tamales once so had a lot left. They worked good and came off the backs of the blocks easy. They measured 10 x 10 1/2. Good luck and show us when you get finished.

When I have used muslin as a base for string/scrap quilts, I have never pre-washed the muslin and never had a problem.

Someone said awhile ago that they used coffee filters!

I pretty much always use inexpensive muslin for foundations, or other light fabrics scraps- I hate removing paper.
I’ve always started in the center & worked my way out but I know a person who just started at an edge/ corner with a 1/2 square triangle then builds from there. I don’t think it matters where you start, just have fun with it.

I have used “used up” fabric softener sheets at times. There is no smell left in them after using them numerous times. Also I just sew strips, pieces together as my leader/ender pieces. Have made over 100 6 1/2 inch blocks. Will need to decide what to do with them at some point. Might be a good project to do with my granddaughter when she visits.

Make your string blocks whichever direction you want them to go. I made 11x6 rectangles on the diagonal. Used a newsprint booklet i got in the mail and just cut the staples off. Liked the size and went from there.