SP:

I Recommend installing the mpClothes addon.
https://www.gta5-mods.com/misc/mpclothes-addon-clothing-slots

Installation Path

All files will go in you mpclothes file which should be here:
mods/update/x64/dlcpacks/mpclothes/dlc.rpf/x64/models/cdimages/mpclothes_male.rpf/mp_m_freemode_01_mp_m_clothes_01

Fivem:


For Classic Mask - Drag and drop nk_classicironfistmask to your resources and start it from server.cfg

For Marverl Rivals - Drag and drop Mask nk_mrironfistmask to your resources and start it from server.cfg

For Both Masks - Drag and drop nk_ironfistmask to your resources and start it from server.cfg

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If you want custom Clothes for your FiveM server or for SP contact with me 

Discord: Nickkonas#7777