In Tezos there are two kinds of accounts: implicit (address starts with "TZ") and originated (starts with "KT"). You can only delegate from originated ("KT") accounts. Moreover, when delegating the entire balance of the account is used. One implicit account can only delegate to a single address.
Magnum Wallet is a multi-currency online wallet. Recently, it added the ability to easily and conveniently delegate your Tezos.
Create a new account (or log into your wallet) at Magnum Wallet. Since the wallet is multi-currency, go to the section where Tezos is stored.
Select a delegate from the list.
Or type the delegate address manually.
Confirm the creation of the KT account and transfer funds to it by entering the password from your Magnum wallet. Press "Confirm".
This may take a while (1-5 minutes).
When the process is completed, you will see a message about the successful operation.