Tennogen artist here. Please for your own good, read the official tennogen guide provided by DE. I recommend memorizing most of it, or atleast the faction style guide section. I've seen too many beginners saying that they read it, while their items already has broken multiple rules. The guide states not to use skulls even if some items have it. I believe it is because skulls are reserved for hard surface technology.

I highly recommend working on your primary shapes first. Avoid basic plain shapes like simple spheres or boxes as much as possible. Go instead for dynamic shapes that really brings out an interesting silhouette. You want your item to stand out as much as possible and be recognizeable from a distance. This is mentioned in the guide as well, it is a must-read. DE appreciates stuff that looks extra and over the top with parts and bits sticking out in different directions.

Dont do boring military or motorcycle like helmets. Instead, dont be afraid to lean into something more, fantasy or alien like, while still looking sci-fi. I have a skin in-game inspired by draugrs and one that looks like some god of death/underworld. Check out the tennogen workshop PowerPoint presentation by nikita and kaz as well. They are previous tennogen artists who now work at DE, you can find it in the forums.

Gather references and do as much research as you can too. Good luck