Plans are a Dashboard feature, and so to charge recurring payments through the API you’ll need to use subscriptions.
You can create subscriptions to charge your customers weekly, monthly, yearly or something in between. You’ll just need the mandate ID of the customer you want to create a subscription for.
If you're using the GoCardless hosted payment pages, the mandate will be created when you complete the redirect flow. The API will then return the customer and mandate IDs, so at this point you can create a subscription or payment for the customer.
You can see a full reference on creating subscriptions in our developer documentation here.