Here's one way I've been sewing the strings. I pair them up by similar lengths, and sew the straightest side together, not worrying about squaring anything up yet. These are pretty rough, but I don't want to have to do too much preparation with this technique.
I press the pairs, then trim both edges straight. I don't make then parallel, because a bit of wonkiness adds interest. You can even things out by sewing the next seams; notice the wide end of the green strip evens out the narrow end of the blue strip.
I keep sewing pairs together keeping the length as constant as possible, but not being too fussy This isn't fabric I really care about, and a bit of waste isn't going to upset me.
I end up with a pieced fabric that I can cut in all sorts of ways.
I used my small square ruler to cut some more 3 1/2" squares for my string cross blocks

and paired up some of the leftover triangles to make crumb blocks, so this isn't wasteful at all.
The blocks end up very random, but that's the whole point of strings, I would think. I've got another block to post soon, I keep thinking of ways to use my pieced string fabric.