I wanted to add this skin to these models, but I have no idea how to

 

You open the .ytd files in OpenIV and you replace the original skins with the ones you want.

If what I just said sounds like chinese to you, I suggest you watch some tutorial videos on Youtube to get familiar with the whole GTA modding and come back to try again.

you would have to replace the texture itself inside the ytd file with the texture that you want to use some of them can be added if they have multi livery support or you might have to replace an existing one with OpenIV