well, to add on to Admin's help, you can also pick textures for your mapmodels.
Here's an example
/newent mapmodel 57 5 5
/newent means new entity, entities are objects
mapmodel justs shows that your adding a mapmodel
57 that's the mapmodel number for platforms, you'll start to memorize mapmodel numbers the more you map
5 the height of the platform
5 texture number, again, the more you map, the more familiar you'll get with these
placing platforms will take some practice, can't wait to try your gema!