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.
Even if you are from the mobile browser or from PC, you can do this thing. Well, the BESY way to get Soul Gems daily and easy is to "Report Spam Comments"! Listen though, don't report every single comment in threads / forum posts or articles, because the result will be an account temp or perm ban. Report those comments that seem spammy on their own for example someone repeats the same thing many times or something to ONLY get XP! That's actually reportable. To tell the truth, in the last 2 days, I've got over 50 Soul Gems by reporting spam comments! Try it out too! Good luck grinding! (1 Successful Report = 1 Soul Gem)