Forge World is amazing in the new Halo:Reach game. It is one, large, massive chunk of beautiful landscape with multiple terrains, indoor and outdoor areas, beaches, cliffs, and so much more, it blows my mind the possibilities that come with it.
The new tools and layout of Forge are a tad difficult for someone who jumps right into it without much prior experience in Forge. I started by building a "Narrows II" map, with a bridge connecting two cliffside bases. I wanted some CTF, but I had troubles making it comply to the CTF gametype.
So, here is what I know thus far on making custom maps for objective-based games:
1) Labels. Label everything objective-based. In the Forge menu, select the game type you want to build with, so you will have the game-type labels at your disposal. Some of the labels, even still to me, are confusing, as there is no explanation with the labels in the Forge game, but some of them I figured out myself. Labels are in the Tool menu ('B' button) when you are hovering on an object, and then under 'Advanced'. For instance, a flag staff would be flag_return (that is the only way I could get the flag to show up). Capture plates I haven't been able to separate from flag staffs, where the flag starts and resets to. Anybody who was able to figure this out, please comment to share with others. Racing, another map type I have played with, has a 'race flag' label to use as checkpoints.
2) Spawn Sequence. Important with racing (not sure about rally, not a big fan of them). Those order your checkpoints, so you have an order in the race. Without a spawn sequence, all checkpoints spawn at once, and any checkpoint reached ends the race. This is also under the 'Advanced' menu with the Labels.
3) Spawn Points. Be sure you modify where your initial spawn points are. The Forge World starts with three of them in the top indoor room of the map, and be sure you delete all those. You can specify spawn points for any team, as well as initial spawn points. You will have to specify a "race_spawn" to start a race with a label on an initial spawn object. If you want a multi-purpose map, so you can use the same map for racing, CTF, Slayer, etc. you can use the next tip.
4) Gametype Item. You can specify, again in the 'Advanced' menu, if the selected item should be specific to the gametype you are editing it or not. You may not want an initial spawn in CTF to be the same as one in Slayer or Racing, so this allows one map to house multiple game types. I haven't messed with this option much, but I imagine this comes in handy.
5) Kill areas. Sometimes, you only want to use part of Forge World (as in CTF, where you don't want the flag carrier to disappear with the flag all the way across the map. Kill areas are "boundaries" you can specify with your map, giving the player 10 seconds to return to the battle field. A great aesthetic alternative to huge coliseum walls or windows, as to not block the gorgeous scenery. This can be adjusted to any length, width, and height you like, and can be a cylinder or box shape. Place these around the perimeter of your battlefield to make sure you don't lose one of your kids.
6) Banks. No, not banks that house money, but triangle-shaped embankments that offer a slight incline and have nice looks. Put these in bases where there are large ledges you'd rather not have to jump over, and these can be great for placing in a corner where you can slowly creep up the bank and peek over a wall that may be too tall for you to normally look over, to get a quick shot or two off on oncoming enemies. These banks are probably the single most important item in a great map, but it depends on how you use them and your map layout.
7) Imagination. It is very common for some people wanting to recreate classic Halo maps. But you could house the next great Halo map in your brain. Be as creative as possible - use the objects in many different ways - flip objects over and see more uses of them at different angles (the circular ramps are awesome flipped over). The new physics properties in Forge allow much. much more flexibility. So have fun!
Note: if you have found any more useful tips for creating maps for special or objective gametypes, let me know via Twitter or comment below!