Ok, I have made a costume before. Heres some details
First off you want some kind of body suit, if possible something like a scuba diving suit, or surfing suit. This may be kinda costly but seriously, you cant make a good costume without spending over $100.
Now you have the body for your alien. You will have to do some research on the runners appearance, I suggest you look at some of the costumes used for the film and fan ones, also have a look at some of Sideshows sculptures of the alien as the show incredible detail.
I would suggest using some kind of solid foam for the ribs and shoulders, don’t use foam that will bend or lose shape as it will be difficult to apply to the body suit. You could probably use the solid foam for some of the head and the pelvic area. Now the more effort you put into it, the better it will look.
The head you will need wire framing to create, chicken wire is great, but is pretty dam sharp in places so be careful. Now this is the head! Don’t use chicken wire to do the neck.
Right once your happy with the shape coat it in paper mache an let it dry, note that the shape of the head should be dome like (as the aliens head is) but when you get to the mouth make it open not closed (that’s the thing you will see out of.) and use foam for the gums and plastic clay for the teeth.
Use solid foam for the dome part of the head (shiny huge bit) nad you have to make it fit perfectly to the head or you will have to make another one. If you really want to get the alien looking real buy condoms. I am serious. Serious as can be. Because you use them for the sides of the aliens mouth were it stretches and looks kinda rubbery. Don’t feel weird using them for the costume either, the people who did the films used them for the aliens mouth.
Now you have the head done you’ll need a neck peace that is attached to the headpiece. Use a cloth like material and attach it so it looks like its part of the aliens neck, you can kinda paper mache it to make it look more rough and alien like.
Sorry I’ll continue more on it , im kinda tired so yeah...