Go to 'Settings -> Integrations'. Select Fiken from the list of integrations.
Add the username and password for your API-user. You first have to enable the API in Fiken.
When authenticated you can select your account in Fiken to connect to.
The next step is to define which bank account to use for the order generated from MakePlans.
And finally you have to link a service in MakePlans to a product in Fiken.
That's it. MakePlans will now automatically add a new customer in Fiken when a new customer is created in MakePlans. You can also manually generate an order in Fiken from a booking in MakePlans.