Sorry, this is probably gonna be a long rambly reply ^^;;;
I’m just in the middle of designing one tbh and I LOVE doing this sort of stuff, but I can’t vouch if my method will be useful to you, since I, well, draw a lot and mostly think about these things straight up visually and draw them hah. I also do less humanoid creatures most of the time so uh… this might not be fit for you? Sorry if that’ll be the case.
But! I think it’s a good idea to start with the biggest things and continue onto details. Silhouette is a good starting point imho, because it can give a LOT of info on your new species in a short amount of time. Are they big and muscley? How humanoid are they? Or small and lanky? Just really tall with looong arms? Top heavy? Bottom heavy? All of these are good starting points. I pick one that fits them best and then go for details. I also like adding a big, defining feature that is easy to spot in this stage (like a mane, horns, bodypart that is in a proportion that is very different from ours, etc.)
And then I start going to detail, and well, this is the first time I start thinking about function. It’s best to have at least some idea about their native planet/habitat at this point. Why would they have their defining trait? (for example, I got mane for my current alien, and they’re a desert dweller, so that shapes what this mane is for. Maybe a water storage? That would have to make the mane be out of fleshy, thick-skinned bits! Ooh, a new unique detail acquired.) I think about how their senses would have to adapt to the habitat, which sense is their primary one, and look for animals that live in something similar, and then nitpick their details, smush them together and then try to tie it all together with something (like skin texture or markings).
As for visual inspiration, I very much recommend looking up concept art for movies/shows/videogames that have a similar feel like your alien has! I also personally keep a folder of bookmarks where I stick wikipedia articles about wacky animals, deepsea invertebrates and exctinct evolved-in-very-odd-ways animals.