It is already set up … see the admin area for access info. As for how to use it - mostly just experience / common sense … but if you are completely inexperienced with similar file storage/transfer/handling solutions I’d suggest reading the material on the NextCloud web site.
https://docs.nextcloud.com/server/latest/user_manual/
This manual is newer than the version of NC installed with MiaB but I would imagine it would still be helpful.
There is also the NextCloud forums but they cover so many aspects of NC that are not included with MiaB.