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 have traded my way into having 12k+ plat relatively easily, buying multiple great rivens. you don't need the plat at a certain point. sell prime junk at 6:12p and that should be all the plat you need