It depends on the size of your human head and whether you had to modify the head or neck to make it big enough.
If you left the head and neck unmodified (and it fits OK), then it is simply a case of making holes in the neck in the correct places for vision, covering the holes with mesh on the inside then using super-glue (crazy-glue in the US?) to glue the top of the neck to the "rim" that runs around the outside of the human head cavity in the Alien head.
If the neck/head needed to be messed with to make them fit you, then it's a bit more complex. I needed to extend the neck upwards with a tube of fabric that included a mesh panel in the front for vision. That was then glued inside the head cavity using contact adhesive (rule of thumb is super-glue for latex-to-latex joins and contact adhesive for latex-to-fabric).
When I say mesh, you can either use fine actual mesh like Simon used for Alan, or you can use 20-denier stocking material like I did. Either way, it covers your eyes nicely in most conditions - your eyes are visible in direct camera flash light.
Another tip that Simon worked out during his build: there are four round tabs on the bottom of the neck at the front, two each on the left and right (about the 10 o'clock and 2 o'clock position, if the front is the 12 o'clock position) that you can glue together in pairs - put glue on the
inside surface of each little tab and stick each side's pair together. This way, it stops the bottom of the neck jutting out as much. This "jutting out" was the reason why Abe wore a collar: to keep the neck under control. After Simon pointing these tabs out to me, Abe doesn't require the collar any more, but he keeps on wearing it!

Whether you tuck the neck into the chest at the bottom is also dependant on if the neck is long enough for you; If your neck is long enough for you to manage it without restricting head movement then it is better. Alas, I'm not so lucky...
Let me know if any of this isn't totally clear (or if you think of a better way of doing it) - it IS hard to make this kit fit together; I'm still learning, and even a crafts/engineering genius like Simon is still working out new things about it!
