Here are tutorials on how to install clothes on FiveM/SP.

For FiveM:
Replace option is replacing existing GTA clothes with the new one. Addons is adding new numbers in your skin creator at the end of the list.

Replace - https://forum.cfx.re/t/how-to-streaming-new-hairstyles-for-characters-step-by-step-for-dummies/1048980
Addons - https://forum.cfx.re/t/how-to-stream-clothes-and-props-as-addons-for-mp-freemode-models/3345474

For SP:
Addons - https://www.gta5-mods.com/misc/mpclothes-addon-clothing-slots

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Credits: Madlen (https://www.patreon.com/madlen)