hey @crisp geyser basically you create a client link and redirect your vendor to the link. a mollie signup flow will appear. if you have a partner account (which you need to use the client link api) you have a new menu item "Customers" there you see your connected vendor Mollie accounts.
after the create client link you add URL Parameters to the link your vendor will be redirected.
redirect_uri, grant_type, code, state, scope, etc.
after your vendor signed up with mollie he will get redirected to your redirect url. there you fetch the "code" from the url parameters. you can aswell use the "state" to verify that it is a redirect that you created. so you would pass a nonce or other unique id to state before redirecting the client.
with the "code" from the url you create a refresh token with: https://docs.mollie.com/reference/generate-tokens
you save this refresh token in your database for the client. and now you can use the api in the name of your vendor by creating an access token with this refresh token.
hope this helps. for me it was also a bit trial and error in the beginning.