It seems like there are so many worthwhile options on DMS Guild - magic spells, subclasses, and other homebrew.  These things in particular once purchased would need to be homebrewed on DND Beyond in order for them to be most useful, i.e. incorporated into the character sheet.

Is there a mechanism for doing this?  If I buy all of the spells in Candlekeep guide on DMS Guild, do I need to then go and figure out how to create each spell myself?  This would not be that hard, but I could not imagine having the technical knowhow to put together a subclass. 

yes you would have to do it yourself, unless the author does it, shares it, and then puts the link in the publication.

edit: assuming no one else has created it themselves though. Very well all could be on ddb already.  You could always start with a search of home brew