How to make a Simple 3d platform for Legacy
By Big Al of Doom Heaven
 

Look at the example wad called platform.wad.
 
1 First create a sector the same size and shape you want the 3d platform to be (sector 1 above), make its floor & ceiling heights the same as the surrounding sector.
I assume you already know how to do this.
 
2 Make a small three sided control sector somewhere near (sectors 3).
 
3 Make the floor/ceiling textures in sector 3 to be what you want for your platform.
 
4 Make the floor/ceiling heights in sector 3 to be where you want the platform to be at its lowest point. In this case floor = 16 and ceiling = 32, gives a 16 unit thick platform floating 16 units off the ground in the main sector.
 
5 Adjust sector 3 light levels so it fits in with the ones in the main sector.
 
6 Tag sector 1 with the next free tag number, in this case 1.
 
7 Also put this tag number on your control linedefs. In this case lindef 2.
 
8 Change the type of linedef 2 to be the solid platform type number 281.
Some editors don't let you enter a number for the linedef type both DETH and DeepSea do.
 
9 Change the texture on lindef 2 to be the one you want to appear on the sides of your platform.
 
10 That is all there is too it.
By just changing the linedef of your tagged line you can create all the other effects like 3d water (301), see through platforms (300), etc. These can be found in the BOOM linedef information available elsewhere.