If you want to play TFT Set 12 Magic n' Mayhem on PBE before its official release date on July 31st, you will need to create a PBE account. It's an easy process and it's completely free of course.
Set 12 will be available on PBE starting on Tuesday 16th so you can actually try the new Set two weeks before its official release.
STEP 1: Download PBE Client
For the first step, you will need to download the PBE client from Riot's website
Make sure to install the Windows or Mac version depending on your PC.
STEP 2: Create a PBE account
For the next step, you will need to create a PBE account through this link
Make sure the region selected is "PBE (English)" as shown in the image above.
Step 3: Install PBE Client
And for the last step you will need to install the PBE client with English (US) settings.
This is extremely important because if you plan to try the new TFT set with other languages than English (US) then you might face many bugs making the game nearly unplayable.
STEP 4: Enjoy!
Don't forget that the Set is still in development and you might face bugs along the way!