I have made tons of these, over about 40 yrs. I beg borrow or steal levis. First I wash them and dry them, than I start at the cuff & cut a little snip on each side of the side seams. that makes 4 snips per pair. Than I just grab the side seam & rip the leg as far as I can to the top. Rip the inside seam as far as it will go also. Take scissors & cut across the top of each front & back leg. You are now left w/2 longer pieces of levis from each leg. You should have 4 long pieces per pair of levis.
Next I lay the strips of levi on the cutting table, making sure that the edges are on a straight line. I than cut the hems off, across. I than pick a number, usually 10 as it seems like a nice round one. I cut each leg in 10 inch pieces. I leave the width alone.
I than serge around each piece than sew them together with about a 1/4 or 1/2 inch seam. Seaming on the 10" side only. I sew them as long as I feel like. Continue making rows than sew the rows together. Some rows will be longer than others, but just cut them off to the length you want.
* the reason I do NOT square up the denim is by the time you sew all the seams together, sometimes it too thick for the sewing needle. My way, the seams are not all meeting at the same time. They are staggered, & it seems to work better for me.
If you have any ???'s please feel free to pm me.
Hope it all helps.