How do I spend the steps that I have banked?

If you are logged into your account via the website, you can spend the steps you have accumulated in your bank by clicking on the 'Spend Steps' button on your dashboard (home page) and then choosing the walk on which you wish to spend them.

If you are using the app, following the advice in this other  'Help' article:

The 'Spend Steps' button on the app is the small circle with the down arrow inside it.