Okay, since I get quite much all the same questions over and over again and I talked about doing one of these a some time ago, here we go:
(if there is something you can't find from there, but would like to know, just ask).
All my wigs are from eBay. There is not anymore the ones left which I used for Rose, Dave, Porrim and Kanaya. My Karkat is this one www.ebay.com/itm/Reborn-Takesh… . Lapirin owns Roxy's so I do not know the seller, but I know that it's from eBay too.
All my t-shirt's prints (Alpha!Dave, Dark!Rose, Default!Rose, Karkat, Roxy, Kanaya) are made with T-shirt transfer paper. Basically it works this way: first you make the print with computer, then print it (to transfer paper), cut the print from the paper and iron it to your t-shirt. Easy as it can be.
Godtier!Rose is slightly different since I wanted the print to be a golden one, not a yellow. I made a template from a contact paper and then just painted the pattern with a golden paint. It took quite much more time than making prints with transfer paper, so if you're making basic print for t-shirts, use the transfer paper.
For face I used Grimas' gray, oil based bodypaint (hands are painted with water based ones). You can also use the water based ones for your face but with oil based you get smoother outcome and those won't dry your skin so much . The bad side of those are that those are really hard to get off (you need some makeup remover that removes oily makeup. I used Garnier's pure daily gel wash). I think you get some makeup remover from the shop where you will buy those paints. It's really important to have one though (So don't think that you will save more money if you don't have one!). The second thing is that you need fixing powder. Without that the oily makeup will not last. You will dirty all the things around you, you need just touch lightly something and there will be gray paint all over it. Though, even with the powder the paint will not last so well, so be carefully. On the powder I used some dry hairspray to get more hold on the paint. It lasted with those quite well but I do not recommend you to go hugging your friends who have white t-shirts :'D.
And remember to try those paints beforehand! Dyeing your whole skin takes a long time (when I'm cosplaying a troll it usually takes something like a 1-2 hours to get ready).
Porrim's tattoos were made with Grimas' water based bodypaints. There isn't really any 'secret' behind those, just take someone who has patience and time to do those with you. When I cosplayed Porrim it took something like 3-4 hours to get my makeup overall ready.
Karkat's horns are actually a fabric ones which are heavily painted on. And I just sewed those horns to my wig.
The base is foil, which I just covered with basic clay. After that I used sandpaper for a smoother outcome. Before painting them I covered them with a couple layers of basic glue (gesso is okay too). After painting, when horns were ready, I glued them on hairband. If you decide to glue your horns directly to hairband, remember to hide the band itself with your wig's fibers so it would look more like the horns would grow from your head instead.
Those are made from soft foam, cardboard, glue, fabric paint, spray paints, two vinyls, sleeping pad, training mat, fishline, gel pen, permanent marker, plastic wrap and acrylic paints.
The top and bottom (red) are from cardboard, everything between them are training mat/sleeping mat. The top white part is also from training mat. The sides of those timetables are covered with soft foam because I needed to hide the ugly layers of training mat somehow. The whole thing is covered with basic glue and after that, painted with red and white. I used kinda interesting materials since I made my timetables in the middle of night so I needed to be satisfied for what I had. Finally, there goes fishline through the timetables, which also keep the timetables hovering in the air.
I made my fangs for Karkat by myself but I have nooo idea what that material is called even in Finnish, let alone in English. It was some kind of plastic which will melt a little bit when you put it into to boiling water. After it has softener you can mold it with your bare hands to the shape you want.
Though, my friends use fake nails for their fangs. I haven't tried those, so I cannot say anything about them.
I use crazy red for Dave, basic black for the rest of them (Rose/Roxy/any Troll in general). I do not have whole eye covering contacts in my Karkat or Kanaya cosplay, the yellow parts are just photoshopped.
For Porrim I used, as well, basic black ones. The colour is just photoshopped to a lighter one. I know that at least youknowit.com has good mesh lences if you wish to buy a pair.