In Tezos there are two kinds of accounts: implicit (address starts with "tz") and originated (address starts with "KT"). Initially, delegation was allowed only from KT addresses but after Babylon update you don't need to originate a KT address to delegate your tez anymore. Now you can delegate directly from tz address.
When delegating the entire balance of the account is used. In other words, any account can only delegate to a single delegate (baker). By the way, your balance is not locked, and you can freely transfer your funds at any time.
Unlike other wallets Atomex is a true HD-wallet, so it is possible to create as many different tz addresses within one mnemonic (one seed-phrase) as you want and split the funds into several bakers.
So, in order to delegate you need to deposit your wallet and choose a delegate (baker).
Choose an address to delegate from (if you have more than one). Fill in the delegate address or select a baker from the relevant list powered by Baking Bad API. Fill in the amount to delegate. Check the correctness, click "Delegate".
Done! Click on "Update" and see the delegation status.
You can also check your account and delegation status in Tezos blockchain explorer.