If you hold tez in the BRD app, you can stake or delegate your validating rights to a baker to earn rewards for validating and publishing blocks. You can do this by following these steps:
1. On the BRD app's home screen, tap Tezos
2. Right below the wallet balance, tap Staking
3. Tap Select XTZ Delegate and then select your preferred Baker from the list of delegates
4. You will be shown a confirmation screen showing the tez you are staking and the Baker's address. Tap Confirm
5. Enter your BRD app's PIN and wait for it to be confirmed
If you wish to delegate to another Baker, simply follow the same steps to change the selected Baker.
Note: When you stake, you are only delegating your validation rights and not transferring your funds. Your funds are not locked and you are free to transfer it at any time.
A baker should be chosen based on the baker’s reputation and performance in order to have some “guarantee” that the baker pays out rewards accurately and timely. This is because the pay out of rewards to delegators is not handled on-chain.