You need to log into the FidoNet website at www.fido.net (enter your email address and password in the top right hand login box) and click "Go". If you are unsure of your password you can use the "Reset Password" link to request a new password by email.
Once logged in you should find yourself at the "Home" page / tab in the Fido Control Panel.
This will then show you an account summary with an outstanding balance
Click on the outstanding amount
Click on "Make Payment"
You will then be asked to enter / confirm the amount
Click on "Make Payment" again
Now choose your preferred payment method / gateway
Go through the checkout process and validate your purchase when prompted for 3D Secure / Verified by Visa authentication
At which stage you should then receive a confirmation that the transaction has completed, or alternatively a message telling you that there has been a problem