If you are doing a fan film, it would be just fine to keep the Predator in his Bio helmet throughout the film. It would be a lot easier to do that than try to do a full head mask right off the bat. It often takes takes months to sculpt, mold and cast a decent mask. But a Bio helmet can take as little as a week.
Here's how I did mine. I've been regularly messaging to offer helpful advice & answer Q's along the way for several other lair members here who'd wanted to give this rather easy method a shot. Most of them seem to find it even easier than they thought it would be!