Step 1 –
Go to the League of Legends website and log into an account that doesn’t already have a PBE account attached to it. Next go to Community and on the drop-down menu select Public Beta Environment, then at the bottom of the page click I want to Help.
Step 2 –
Check the requirements on the “Apply to be a Tester” page and make sure that you meet all of them, if you do click I want to be a Tester at the bottom. Your application will then automatically be submitted and Riot will review your account to make sure you meet the requirements. Once your Application is reviewed Riot will send you an email with the results, and if you are accepted there will be a link to create your PBE account in the Email.
Step 3 –
Once you have the account you need to download the PBE client which can be found here. Then you just log in with the PBE account you just created and you’re ready to test. Keep in mind that PBE accounts are pre-leveled to 30 and come with quite a bit of IP/RP so you can literally test anything in the store as soon as you load in.