The next step is to start gluing that monster together.
Like I stated earlier, I use plain old super glue for the job. It's dries in seconds, so just make sure you line the pieces up properly.
Remember when you labeled all the pieces into sections? (Right forehead, left cheek, etc...) That's where you start to appreciate your efforts. Sort all the pieces into their respective little piles. Really, you can start gluing any pieces together that you like, but some pieces will be easier to handle before they're all glued to another piece.
NOTE:
Something I learned while putting this together was that you should probably start on the most detailed parts. For instance, on this predator mask I assembled all the main parts, and built them around the visor piece last. This made it hard to fit properly to the visor. I should have started by gluing the face together first, ending with the easy large pieces.
Sometimes as your putting it together, you might wonder why it doesn't look quite right. Just keep going. There will be a point when you connect two pieces and it suddenly comes to life. Just keep plugging away.
Notice how the left jaw is a bit wider than the right. That's because I missed one little tab. After I glued that one...
Now that it's all done, take a minute to enjoy your hard work. The next step will be to make this thing permanent.