I made a 944 skin on my XRG and I have the file and everything but I can't seem to use it because it's not on this website, how do I solve this (Demo user)

You can add credits to your account which you can use to buy skin slots.

You can buy 50 extra slots for 1 GBP on the skins upload page. There should be a button saying "Add 50 extra slots". Once you do this, you will be able to upload skins to your account and use them online.