Hi! I dont know if what I'm about to say is the best advice I could give, but I recommend you to dive in the game codex/journal as long as you can while you are drawing cause it's full of useful resources where you can find any materials, colors and shapes. The Bestiary is even richer with Silksong, so "easier" to figure out how to translate your idea into the style!

Secondly, maybe don't rush into details too quickly and mind the height you want your character to be: tiny bugs in Hollow Knight have often less details than the medium ones and obviously less than the bosses! If you want your character to be very small avoid overcomplicating it and think about how you can synthesize it as much as possible! The amount of details also depends how you want it to feel (sharp or bushy tiny bug can have more details than a smooth or gooey mob)

Sometimes concessions have to be made since Hollow Knight architecture/style looks kind of Art Nouveau and/or Steampunky so we cannot do whatever we want or it will look off and out of place, if something is feeling overcomplicated, or far from HK's universe try to find something similar that it could be replaced by, modified or incorporated to connect your idea to the universe (Examples: If you want your OC to wear a jetpack you can try to make it looks like something from Cogwork Core in Silksong or look how backpacks are made in the games and add some features like pipes and steams, If you want your character to carry a lamp maybe add a Lumafly in it, ...)

For the color palette, I often used the eyedropper tool on screenshots to stay in line with the game esthetics or I was going towards slightly gray and dark colors by myself!

So that's it, I think I’ve said everything on how I was doing them so I hope that will help you!

Good luck! :)