I have a few suggestions. The first and foremost is to refer back to texts that have descriptions of monsters you find very appealing or match what you're imagining. See what they do. I always encourage reading texts that fit within the lineage/idea sphere you're writing in. Perhaps look at the description of the Creature in Frankenstein; seems like a good place to start. That said, sounds like you have a decent idea of what you want your monster to seem like--and depending on the function of it in your narrative, you may not need an extremely detailed description of the monster. Just enough to make your readers feel unnerved. Think about what would make a person seem uncanny to you. "It looked like approximately like a human, but its head was slightly too small, its hands slightly too big. The elbows were in the proper place, but bent the other way." Something like that?
Second: Engage in more craft/theoretical texts around writing. I would look at Writing Monsters: How to Craft Believably Terrifying Monsters to Enhance Your Horror, Fantasy, and Science Fiction by Philip Athans. There's a useful monster creation form in the book. He covers a lot and I think it's particularly useful for helping round out your monstrous ideas.
As for my own process, I tend to write kaiju fiction, so my design process might be fairly different than other horror/scifi writers. Kaiju is often "monster first", so I start with a monster and then figure out where I go from there. These are often vague impressions of a monster and usually based around a pre-existing entity or concept (a giant alligator or "What if Ultraman was a dragon?"). Then I think about what the monster means. I'm of the camp that monsters should have meanings (after all, "monster" comes from the Latin monere, "to warn, to show, to instruct"). What idea/theme am trying to communicate with this monster? This doesn't mean it has to be something didactic or on the nose, but the monster should do something more than scare the protagonists or reader, I think. That then helps me shape the monster's narrative function and characterization. As I develop those, the form becomes more concrete because it has to achieve certain narrative ends. I want my Ultraman-Dragon to be ambivalent, with characters debating whether he's good or bad? I use descriptions that meld both threatening descriptors with heroic ones.
A final tip. I've started writing out my monster's designs outside of the story. First I write a neutral description. Then I write the description of the monster when certain characters first see it. Different characters will apprehend the same monster differently. One sees an elegant dragon, the other sees a world-destroying serpent. A sane man sees a quivering mass of tentacles and shapes that should not belong on this earth; the insane man sees the face of God.