making a mold help

ok so ive carved out a large ufo out of foam now i wanna make a mold out of it so i can make the body of it out of fiberglass the body of this ufo is almost 4 feet tough how the heck do i even go about pulling this off if any one could run me through a good step by step process i would greatly appreciate it

If it's a simple shape, and you only need one casting, you can do it without a mold: apply fiberglass directly to the foam. When it's set up, use acetone or other solvent to dissolve the foam, leaving the fiberglass shell. This "lost foam" method is quick and cheap for one offs.

If you need a rubber mold for multiple casts, check out Steve's Enterprise thread. The saucer section should be very similar to your UFO.

That lost foam method only works with styrofoam. If you used a urethane foam, it won't dissolve, but you can painstakingly carve it all out. Been there and done that. If you are trying to get a lot of detail on the surface, I would make a mold, and if you don't have any undercuts on the shapes, I would make a thin fiberglass mold. Laying glass on top of your foam alone will lose a lot of detail, and make it harder to get a really nice surface fiinish.