If what you are trying to create is easily accomplished by reflavoring an existing, official component of the game. Do that. Not only is designing it yourself wasted time and effort, but often will be poorly balanced in comparison.
Ex: If you’re looking to create a massive serpent that spits poison, simply edit a green dragon and adjust its challenge rating accordingly.
As a rule of thumb, stick to established design format. It’s there for a reason.
You are free to go against status quo, but keep in mind this will require extensive playtesting, and will generally get your homebrew dismissed by curators.
When formatting your homebrew text, try to find the middle ground between Comprehensive, Traditional and Concise. WotC’s wording of certain spells, classes, etc. is poor. Don’t take their work as gospel, but try to keep your formatting familiar and to the point.
When designing player options, be extra diligent in your attempt to balance your new material against published works. Aim for below average power, but powerful enough to at least be attractive for a player. It’s easier to give a class or race something extra if it turns out too weak, than it is to take features away if it turns out too powerful. Especially if your players are already using them! Imagine how you would feel getting something extra against having features taken away from you.
A homebrew class should grant proficiency in two saving throws at level one. Choose one from [Strength/Intelligence/Charisma] and [Constitution/Dexterity/Wisdom]. A player character with only one class should never feel pressured to spread its strengths too thin. Beyond the every important Constitution score, a specific (sub)class should never focus on more than 2 ability scores at once. For instance, Rogues tend to perform best on a combination of Dexterity, Intelligence and Constitution, but certain subclasses might substitute Intelligence with Charisma.
A class’ hit die should be proportionate to both its flexibility and its general approach to combat. Classes designed to be at the front most often have larger hit dice, and classes that focus on staying in the back have smaller ones.
Constitution is *not* a spellcasting ability.
Don’t give your class too many resources to pull from. If your character has spell slots, give them one additional resource at most. Ex: Sorcery points.
When creating a subclass, attempt to have the its features fill the same niche as the ones from existing subclasses for the class. If a feature focuses on broadening the player’s options in exploration, consider having your subclass’ feature share that focus.
NPCs have a proficiency bonus related to their challenge rating. Whenever a creature has a feature with a saving throw, that saving throw’s DC equals 8 + the creature’s proficiency bonus + the creature’s relevant ability modifier. Exceptions to this rule exist, but only rarely.
Always ask for input from other experienced homebrewers if you intend on going public with your homebrew creations.