Go to the ENS Manager App with the wallet set as Manager for the ENS name you want to manage and click Names to bring up a list of your ENS names or search for an ENS name you own directly from the main page.

Click the ENS name you want to add records to.

Go to the Records tab

Click Edit Records

Click Add Record

In the new field that appears, type in the name of your Custom Record. Records traditionally use a reverse notation, e.g. com.youtube for youtube.com and then click Add

Type in a value for your new record, and click Save

Then click Open Wallet and approve the transaction in your wallet.

Wait for the transaction to complete.

Once the transaction has completed your record is stored on-chain and should be immediately accessible.