You have to have the "underlying" block device active first. So, you have to have your drbd device all created and available on the host first. I don't know if drbd devices are supposed to be partitioned or not but do as is usual for drbd.
After you have your drbd block device available on the host, you can start working with the cryptsetup tool. It will take the drbd device, run its thing on it and it will create a new device (/dev/dm/something) which will be the plain-text equivalent of your device. This will be the device that you will create filesystem on and that you will mount.