I have a few ideas but depends on how much effort you want to put on it. Diy is not only a punk feature, goths have always diy'd their clothes. I am more punk than goth but the two are not that far! They are like alt cousins :)
Quickest and easiest way to make a pair of sweatpants look goth is to bleach them. You could either dilute it and spray it on the pants or put a bit of bleach in a container, take a brush and just splash it on the pants. Leave them outside in the sun for a bit and wash them (do not put them with other clothes just to be sure you are not ruining anything). I'll say one hour and you have your personal pair of goth sweatpants.
Another thing you could do is to put holes in them. Take a pair of scissor, cut a small hole and then bush a razor on it to give it a more ripped look.
If you have more time, a bit of money and you want to try something more complicated, you can sew patches, paint them, add some metal eyelets (idk if that's how they are called in english. The metal things in the shoe where you put your laces in), chains, etc.
As for the top, band shirts!! Black shirts!! Black tank tops!! Halloween themed stuff!! You can also find them already goth enough or add patches, pins, laces, etc etc. Same goes for sweatshirts! Be creative! And also accessories!! Spike bracelets, necklaces, etc can change the look and make it more goth even if you are just wearing a pajamas.
Definitely look at old goths looks. They can be the source of great inspiration for diys. You do not need any fancy or expensive equipment or that much experience to make cool things. And remember that all of these subcultures are about breaking societal norms and expectations. So honestly going around in your pjs sounds very goth!!!
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