The Construction Kit is, in my opinion, one of the best things about the whole game. With it, you can make new maps, you can add trees, water, craters, units, and buildings. You can add endconditions and fsms( they're explained in the advanced Construction Kit page. The people at DarkReign even made THEM easy!). The basic way to win, without endcondtions, is kill all enemies. This tells you how to make a map like this

How To:
To put units on the map, click on the man on the toolbar, to display the units of the selected group, you change that group by clicking on the number pad at the bottom. To put buildings on, do the same thing, except with the house instead of the man.
To put water or taelon on the map, click on the house on the toolbar, which displays all buildings, click N on the number pad to select Neutral group, and click on the water/taelon.
To put overlays on the map, click on the tree.
To make a single/multiplayer player mission map:
Click on create and type in the width/height of the map (in tiles... the default is 120/120 and it almost fills up the mini-map)

Change terrain with the 'mountain' icon on the toolbar. Check the two boxes to change what you put on. One puts on only terrain, with no altitude, and the other, when checked will put only altitude. You can check both or neither (though I don't know why you'd check neither!)

To change the 'fsms' and 'end conditions' (explained in the Advanced Mission Page, go to the checkered flag on the toolbar.

To change a player's credits, start position, and alliances, click on the 'head' on the toolbar.

To make a single player mission, you have to have the odds against that player. Most of the time, even with the hard computer, humans will win, and the computer will lose. So you have to have them at an extreme disadvantage. Goodones would be to have the computer start off attacking the base(without programming the only way is to put units inside the player's base). Or you could make it so the player was FG and the computer was Imperium or SH and have lots of water on the map, so the FG player would have more trouble hovering. You could set the players credits lower, give him/her less water, or let the computer have allies. Any way you want to really.

To make a multiplayer map, it's easiest to just start everybody off equal. Otherwise the missions are very hard to balance.

You can look at the statistics for the mission by clicking the last icon on the toolbar.

Go to page on creating ADVANCED new missions (programming involved).