How would I go about adding in custom textures?

I’m wishing to install something as such: https://www.gta5-mods.com/misc/sahp-office-paleto-bay-mod. However, I’m not quite sure how to install it. I ran across this while making what I’m typing right now. Would the response given be accurate? And if so, how exactly would I go about doing so (I’m not the smartest cookie in the jar). Any help would be appreciated.