Play games
Choose the game that interests you and play itfor free.
Complete the quests
Every game has some quests for which you will receive Soul Gems.
Receive rewards
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
I know it's crazy but take it from the other side. First tanks of every country were usually used againist Infantry. Weak armor but still enough strong to stop bullets from guns, only weapons are heavy machine guns, no cannons...