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.
You can exchange your Soul Gems for Steam Wallet top-ups, game keys, CS:GO Skins and other rewards.
it tells me In order to open this chest, you need to collect at least 100 SG from the time you last opened a promotional chest. I already earned 506 SG and I can't open it