Here you can see an option to add validator (Validator with + symbol).
Click that option and a new pop-up will come like this :
Here you have to select your account (SS58 that you added) and the value which you want to bond, minimum currently is 1000 $GPU i.e. the staking amount.
Click on next and you would be able to see the following window:
In the 'keys from rotatekeys' section paste the key that you created in the previous step. Click on 'Bond & Validate'.
Then enter your account password and select 'Sign & Submit'.
Note: After this process, you should be able to see your account in the pending validators. However, your validator will start making blocks and earning rewards from the next era (1 Era is 24 hours).