First of all, you have to be in a your Guild must have at least Arena Level 1.
Click on the Guilds tab
Click on the Brawl tab
There will be a timer indiciating when the next Brawl starts:
Once it hits zero, there will be a list of Frays displayed.
Each Fray has its own ruleset, i.e. Novice level caps, only Untamed cards allowed, only Gold Foil cards allowed, etc.
Click on Enter Fray, and you're set.
Once the 'Brawl Begins In' timer hits zero, you'll have a list of Battles.
Submit each battle before the end of the Brawl, or you will be scored with a Loss.
Each win will give your Guild 3 points.
Note that if you lose possession of a card before your opponent submits their battle, you will be scored with a loss. This includes rental expiring, renting out, delegating, selling or transferring.