First of all:
I hope you have already joined the seals, if not, be ashamed and do it now.

Tutorial:

In Open4: Go to GTAV< mods64v.rpf< models< cdimages< streampeds_players.rpf< player one and drag and drop the "uppr_diff_004_o_uni.ytd"

Now in game: Go to closet with Franklin and now t-shirt. The shirt is called: Pounders

Join the discord: https://discord.gg/TZE4ka