I'll hopefully have some time to put together some better information on subclasses soon, but for now here's a start!

Here's a quick tutorial put together by Hartlessd20. 

• If you wish to publish your subclass, but are unable to because the website tells you that your subclass is too similar to another class, please make sure that you have completed a full description of the subclass.
• If you are told that you do not have the correct features at the correct levels, please consult the table below. If you feel that you already have the correct features at levels, please make sure you hit the save button on your class after adding those features.

Remember that eligibility to publish is only re-checked when you hit that save button!
If you are seeing a message in red text that prevents you from publishing your homebrew, but believe it is incorrect, please hit the SAVE button.

Features at the correct levels

The core design of D&D 5th edition ensures that there is a balance of features at each level for a player.

What this means is that each core class has gaps in their features - these gaps are where the subclasses for that class provide their features.

D&D Beyond adheres to this philosophy, so it is important that you do when you're designing your subclasses, if you want to be able to publish the homebrew for public use!

You may, of course, add whatever features you like to your subclasses for personal play and sharing privately via a campaign - this rule is for published homebrew.

The table below shows what levels each class has gaps in the features, where you should place features for your subclass:

As an example, let's take a look at the Fighter class table, where you can see I have marked the entries for Martial Archetype and Martial Archetype Feature. These match the table above. Any Fighter subclass designed will need to have features at these levels to be valid for publishing, further if the subclass has features outside of these levels, that will also disallow it from publishing.

Spellcasting?

This section can cause some confusion.

For classes with no innate spellcasting (Barbarian/Fighter/Monk/Rogue) - make sure to set this to NO unless you are adding spellcasting through the subclass, at which point you should set this to YES. If you leave it blank, it may cause issues.

For classes that do have innate spellcasting (all the other ones) - make sure to set this to NO or leave it blank, as you aren't adding spellcasting.

Also note that, for a subclass to add spellcasting to a class that doesn't normally have it, you will need to add a subclass feature SPELLCASTING.

This thread is currently locked because I will be adding more content to it and don't want to have to delete your posts. :)

Artificer added.