To change skins, it will be best to have a Factory New skin for the weapon you will like to change the appearance of, don't spend too much money on this as it is only for offline games.
To change the appearance of stickers you will need any sticker in your inventory and then once you have edited it, you can 'Apply Sticker' and see what it looks like on weapons and skins then, but don't apply it because even though it looks like what you edited it to, it is still the original sticker if you change the files back. This helps especially if you want to buy a skin or sticker but want to see what iot looks like first. In this guide, you will be told how changing the files to change skins and stickers works and how to do it.