Also, go into your Halo CE Directory (C:\Program Files\Microsoft Games\Halo Custom Edition) and there will be a folder called HEK_Tutorial.
Inside is an HTML document that contains a step-by-step, picture and text tutorial of how to build a map that was put together by Gearbox themselves. It contains info for making BSP Portals, Water Planes, and even explains the basic functions of 3DS max as well for complete beginners.
Definitely highly reccomend. If you need video assistance as well, Sparky is pretty knowledgeable so I'm sure his video will be useful; Link