well first off i do want to say (before explaining a long process) that they do sell fake scars and such online if you search, though coloration and length may be different than what you need, now to MAKE fake stitches.
You will need,
a flat SMOOTH surface at least 2 to 4 inches longer than you want your scar to be
a make up sponge
the needle and thread you want to use
grease paint for the skin and wound coloration.
a hair drier/ heat gun on cool settings (optional)
1) what you need to do is lay a thin layer of clay, maybe about a 1 centimeter thick on the flat smooth surface and sculpt the wound. it will need to be long and rounded, the thickest point in the middle of the scar, where you will cut out the wound, then blending out to the thinnest around the edge.
2) you will use what ever tools you have (could be a razor blade or a letter opener, what have you) and cut the long straight line to the wound to carve out the actual open wound stop about half an inch before the end of your base sculpt.
3) to smooth out any roughness run some sort of heat over it to melt it GENTLY! too much heat and it will all melt and you will have to wait for it to re solidify and try to fix it, if you have a small torch pen on the lowest setting would be a bit much still, be very careful with open flame! if you arent allowed to use fire, try a heat gun/hairdryer on a hot setting.
4) now that it is all nice and smooth you can start with your latex! using your make up sponge dab on the liquid latex in small layers, start with the middle (the wound area) and lay a layer their, let it dry FULLY! you can speed this up with a heat gun/ hairdryer on a cool setting to blow air on it. you would then extend the layers larger over two or three more layers ( so that it is thicker in the middle and thinner at the edge, like your sculpt) and then build up more and more layers, about 10 to 12 total should be fine.
5) now that it is all dry! ( i would let it dry over another hour or so just to be safe) powder it with baby powder so that it will not stick to itself when you are done. you will then CAREFULLY lift up an edge with a paint brush, any kind is fine, make sure to use the paint brush in combination with the baby powder to lift up the edge gently, try not to rush, its a delicate process that could poke holes in your prosthetic, the clay underneath should actually leave a layer on latex so you shouldn't need to powder it.. but powder anyway. better safe than sorry.
6) now that you have that done, color it! using your grease paint or your preferred stage make up color the latex a flesh tone and the center of the wound a more pink color as it is shown on the model you provided. set the color with colorless powder/baby powder, though baby powder may change the color of it when done.
7) almost done! take your needle and thread and ACTUALLY sew it! since it is rounded with the center cut out it has that lip that you can pinch together and really sew it for a very real effect or since your model looks like it is healed, when sculpting just sculpt it slightly open and not fully and paint on the heal marks from the stitching.
annnnd you're done, now this is a professional method and takes some time and work, but is well worth it and is able to be used multiple times to apply it you can use spirit gum ( i prefer Pros-Aid, a type of medical adhesive able to be ordered online for about the same cost per ounce as spirit gum) and brush on a layer of it onto the edge of the prosthetic and have a friend help you apply it to your side in the area you need. I would recommend placing it where you want first and use some light blush or something and put it along the edge, then when you take it off, you will have that guide line of blush to position it exactly where you want. make sure to blend the edge of your piece as well as you can, going to youtube and searching for videos on this will help, there is quite a few of them, for removal you can use a high proof of alcohol and cotton, rub it on the adhesive and it will melt it slowly. gently peel it away, it will stick very well and might hurt a bit, especially on any hairs you may have. this is a bit costly if you dont own anything you need, the liquid latex its about 10 dollars i think, the clay is about 6 the grease paint is god knows how much for the colors you need, but expect at least 15 to 20 dollars for two colors. but like i said they sell PREMADE ones online and is much cheaper and is the same process to apply it.