Creating a good D&D 5e homebrewed monster is a matter of balance. Like all things within the bounded-accuracy system of fifth edition, most of the specific numbers for attacks, armor class, and attributes are unnecessary. What you need to know is how to land within certain likelihoods for damage input and output, understand how to balance the action economy of fifth edition, how the difficulty of a fight is set, and the purpose of legendary/lair actions. While this seems like a lot to keep in mind, most of it falls under a series of general rules that make it easy to adjust a fight as it happens so you wind up with what you, the Game Master, expected.