Step 1. To get started, go on an official website Magnum Wallet
On the first page press the IMPORT/RESTORE button. Then enter the existing private key and the passphrase, press IMPORT and download a wallet backup.
Step 2. Tap on your Tezos wallet and choose delegation option.
Previously created KT-addresses are shown as a grid at the bottom of a wallet page. You can tap on one to see detailed information about a baker.
In case you want to create a new KT-address select a baker from the list or insert address manually. Enter the amount you want to delegate and press NEXT. You need to enter your passphrase (from Step 1) and press CONFIRM. You will be charged approximately 0.001 XTZ and 0.257 XTZ for the network and storage fees which are required to create a KT-address and to send a transaction.
Also, you can see all delegated XTZ and your bakers in the wallet.