You’ll both need to have Monzo Personal accounts before opening a Monzo Joint account.
When/if(!) you have a Monzo Joint account opened, you can switch an existing (non-Monzo) Joint account to the Monzo Joint account. This is a ‘full-switch’ and will transfer everything over and then close the non-Monzo Joint account. There’s a run-down of the in-app steps below for extra clarity.
Disclaimer - I’ve not used the CASS method. I manually switched everything across to a Monzo Joint account several years ago, simply because I didn’t want the source accounts to be closed. It’s not as big an issue as you’d think to manually transfer, but it does take a bit of time & concentration.