The best and easiest way to get free skins in CS:GO is to have the Prime status upgrade to your account. Surely, you have to pay for it first, but it will multiply your investments indefinitely as long as you play, play and play! Prime's profile levelling system is what gives you free skins.

Level up each week. One of the easiest ways to earn free skins in CS:GO is to just play the game. You get a loot drop after your first level-up each week. This is usually a drop containing skins.