You need to be a Snoop Plus user to access this feature…
All you need to do is open the Snoop app, then tap on the '+' add account option and select 'Add an offline account'. You can then choose a provider (if applicable) or scroll down and select 'Provider not listed?' to create your own.
Once created, you can click into the account info and can add notes to the account, update the balance, add interest rate info, and add a scheduled balance update.
To delete an account, tap the three dots on the right-hand side of the account info page to change the name or delete the account.
Snoop Plus is available for £4.99/month or £39.99/year.