I worked at a Sheltered Workshop as a Supervisor in a Textile plant. Teaching our Clients to sew was a challenge at times.The sewing was done with older industrial machines. No frill like zig zag or moving needles. To mark our seam allowances we cut a 1/8" thick piece of painters tape or electrical tape and mark the machine. Cut it about 4" long being careful to place it straight on the bed of your machine. The fabric slides along that 1/8" ridge. You get a perfect seam.