How do you add a race to a race-optional monster?

Question

I am wondering whether or not you add in racial ability increases to the already stated stat block, for example, in the assasin monster?

If I wanted the assassin to be a human, for example, do I need to increase all of the ability scores by one or do I make it a variant human and give two ability scores a plus one and then choose a feat?

Or do I just consider the already listed stat block to have those increases and would only add in other racial things such as dark vision and languages for a Dark Elf?

Is there anywhere in the DMG or the Monster Manual stating what to do with these race-optional monsters?

Answer 1

There's a table of racial features and some explanations of how to add a race to an NPC on page 279 of the DMG (the chapter on making custom monsters). That information itself is an expansion on the customizing NPCs options shown at the top of Appendix B in the Monster Manual itself.

Note that the monsters there already are humans, so no change is needed to make them humans. But you can apply a few quick ability modifiers and some traits from the table to quickly turn them into members of a specific race.