You can definitely look up tutorials on youtube, but you just sew wefts right on too of the wefts in the wig. I’d suggest doing it on a canvas head that’s the size of yours so the wig doesn’t shrink up your from sewing. And yes i get the hair from Amazon as well.
https://www.amazon.com/dp/B072HH3FCK/ref=cm_sw_r_cp_api_i_41gzFb3RYXWVF
That’s the hair I’m getting to add into it. Now the hair i KNOW is good because I’ve bought it twice before lol. I love it. And as far as adding the hair of you want it to just be a little thicker, you can sew on top of every other weft in the wig or every third weft and that should be perfectly fine for just more natural thickness or wherever suits you. And of course if you want more you can just go back and add more 😊