First off, don't double post, just use "edit".
Use sparkedit to put in scenery/vehicles/weapons. I don't know how to change projectiles but there are tutorials in the tutorial section.
Please actually TRY to find them by yourself. Don't ask people for questions that have already been answered...it's basically spam.
Also get the full Halo. It's only $10....seriously. You can't mod with the trial version that much anyway.

I am the 14,229th member to register at halomods. And 14229 is my lucky number.