Here's a video on how to use forge in Halo 4 prepared for you by the team that made it, Certain Affinity (who are no strangers to helping out on Halo related content).
In their words:
This video covers the basics of editing using Forge in Halo 4. We'll quickly build a simple Mongoose racetrack using Duplicate, Magnets, and Rotation Snap. People new to Forge should leave with a better idea of how to use Forge, while those more experienced will get a taste of the new features that make editing in Forge faster than ever before.