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.
You can find the currently ongoing Steam game giveaways on this subreddit: https://zz.reddit.com/r/FreeGamesOnSteam/ Just read the comments for every post, sometimes it's not clear what you have to do to get your key. Have fun.