In order to set up a direct debit to pay funds into any of your accounts, first login to your Octopus Money account, click on the account overview for the account you want to set up the direct debit for (where it shows the amount in your SIPP/GIA/ISA).
At the top of the screen, you'll see a 'manage funds' button.
Select this and follow the instructions to set up a direct debit, picking the amount you want to pay into your account and the date of the month you'd like the payment to be made. For regular and one off payments, there is a minimum amount of £25. For security reasons, you'll need to verify your bank details first. Follow the instructions on the screen to do this.
Tip: Not seeing the right thing in your account? A hard refresh will reset your account, so you can see the latest version of your account. Instructions to do this on Mac are here, and on Windows are here.