If you are looking for tutorials and FAQs, you should first check out the Everything you need to know about Homebrew (Tutorials & FAQ) Forums thread. This thread contains a series of tutorials on creating homebrew content on D&D Beyond, followed by a FAQ post.
There is also an excellent set of Design Workshop articles written by James Introcaso:
- Design Workshop: Backgrounds
- Design Workshop: Feats
- Design Workshop: Magic Items
- Design Workshop: Monsters
- Design Workshop: Races (Species)
- Design Workshop: Spells
You should also familiarize yourself with the Homebrew Content Rules & Guidelines before creating your own content.