Make sure you have Notepad++. Check Blizzard.j and Playerslots.fdf in War3.mpq and War3x.mpq, in Blizzard.j you're looking for "Starting Units"
Make sure to look closely at how the PlayerSlots.fdf in Nirvana.mpq is set up too. (PlayerSlots.fdf can be opened with Notepad++)
After that, it's just converting your map files using a Widgetizer and putting everything into an MPQ using Ladik's MPQ Editor. That part I can't figure out though because you need a listfile and there's no easy way to make one other then writing up the entire list yourself. (Which would be a huge waste of time)
Of course, put all your models, sounds, and other files in the MPQ as well.
Finally, open your map with an MPQ Editor, and copy everything in MapScript.j and put it in Blizzard.j. Everything in MapScript.j will be all your triggers and variables. If you want Globals changed, make sure you copy thoughs out of MapScript.j as well.
And one last thing, you need to make a text file, put this in it: Nirvana.exe -mpq Custom.mpq -race YourRace1, YourRace2 (Replacing "Custom.mpq" with your MPQ, and YourRace1/2 with your race's name. This is also confusing a bit. I don't really know everything about it yet and that's one of the things holding me back on my mod.)