How can I make a audio sounds like talking inside a space suit?

Absolutely dry recording, and EQd so that there's a mid-range boost (caused by being inside a helmet) and most of the high frequencies rolled off above about 5kHz - that's to simulate a communications-style microphone, which is what they would have been using anyway. The slight mid-range boost would have to be from about 800Hz to about 1.2kHz at a guess - you'd have to experiment with that a bit. To make it truly realistic, you'd need to add just a touch of distortion as well, and probably put a limiter across the final result.