The process of setting up a Mollie account for iDEAL payments is described here:
Connect the mono shop
Once an account has been set up, the mono shop must be linked to your payment gateway account.
This is done here in the mono editor: SHOP -> PREFERENCES -> PAYMENT
Select the iDEAL payment gateway, and then input the information needed to connect the shop:
For iDEAL payments you need to insert the “Partner ID” and "Profile Key". This information is found in your Mollie account.