The resource is called LEDI. Assuming you have the templates that you need (which I believe can be copied out of the Avara application or most sets), then it will be self-explanatory as Tundra said. In case it isn't... Put the resource name of the SS... PICT in the "Path" field, and set "file" to false (this tells it that the level is found within the file and not in a file in the same folder named firstlevel.pict or whatever).
For the rest of the fields ... in "Tag" you should put four unique characters (this allows the player file to store record information). "Enables Required" should be set to 0, unless you want the level to be invisible until another level is completed (as with a series of solo missions -- you have to complete EnablesRqd number of missions that point to the given one in their "Enables" fields for the level to become visible). I always set "Reserved" to $00000000 -- I don't know what it does. By the way, you should be using ResEdit or some such program here; the file should be of type AVAL (avara level) and creator AVAR (avara).
Rebel... last I checked, a bug in Apple's system of copy-pasting PICTs makes setting wall heights individually (using rounded corners, I mean) somewhat risky. Use the wallHeight variable, which will change the heights of all walls placed after you set it, until you change it again (you can also make it point to other variables (e.g. kTwo) so that you can tweak things like the height of your level's second story in a single place at your level's header (or even in a TEXT resource) -- see "ballgame" for an example of how to do this).
