You need to be logged in to your Ladbrokes account using the Racing Post Bet Slip. You can then use the deposit feature, located in the account section.
The last 4 digits of your card number will be displayed. You will need to enter the CSC number (the last 3 digits on the back of your registered card), how much you wish to deposit and your account password in the relevant fields. Scroll down and click 'Deposit'.