Here is a very basic tutorial on building stitched scars.
Plexiglas Sheet, ceramic tile or ultra smooth, portable work surface
Inexpensive soft paintbrushes (you'll probably ruin them and throw them out)
Opaque foundation that matches whoever is going to wear the finished scar
Heavy black thread and a sewing needle
I made a worm of plasticine and smoothed the sides of the worm into the work surface with my fingertips. I dragged a sculpting knife all the way down the high point of the worm to create a gully. I made a "Y" shaped scar, but you can make any shape you'd like. I used a soft paintbrush to coat the scar with the Mold Builder. I gave the scar a one inch border of latex that will taper onto my skin when I apply the scar later. I gave the scar another five coats of Mold Builder and let each coat dry completely before applying the next. Make sure to wash the paintbrush out immediately with warm soapy water. If you leave it too long the latex will set in the brush and you'll never get it out. I bought six inexpensive brushes so I wouldn't be bothered if I ruined them.
After all six coats have been applied and are dry, I dusted the whole scar with some corn starch to seal the piece and prevent the piece from sticking to itself when I peel it off the work surface. The corn starch also helps the makeup to stick.
I added a wee bit of opaque foundation and dusted it with cornstarch again to set it. This is just a base coat and it will be scratched up when I apply the piece, but it will help with building depth in the finished look.
I sewed through the two edges of the scar with the black thread. I tugged gently to create a bit of puckering. Dip the sewing needle into the cornstarch before each stitch to help the needle go through the latex more smoothly.
I added some fake blood to see if the piece looks convincing, but I would leave this step out until I have applied the piece. I just wanted to see if the design needed any tweaking. I think it'll do! I'll post a tutorial on how to apply the scars closer the Halloween. I'll be using spirit gum to apply the piece and I'll show how to blend the edges of the piece into your skin.