If you want to fly the Danish flag, you have to apply for certificates for the vessel.
Nothing is needed.
Safe Mannning Document – Danish Maritime Authority (only works with Chrome): https://secure.capevo.net/xform/frontend/irs.aspx?alias=BSF&groupId=456&casefolderid=598&doctype=1&__CustomParameters=LanguageShortName%7cengel&formid=14161
In straightforward cases, you can expect to get an answer from the Danish Maritime Authority within one month.
The Danish Maritime Authority will give advice on how to fulfil the requirements.