It's just a text file with a list of cards. I think it's one card per line followed by x1, x2, x3, ect. It's nice to throw in comments to create groups, such as //Creatures. Then there's a special tag where everything after that line is the sideboard. Again, I don't actually have Apprentice, so I'm just saying it all from memory. It's real simple, look at the default ones and the ones Apprentice saves.
SHAD0Wdump, it would be a big help if you sent decklists formatted properly to begin with. Then you could just tell people something like:
Copy and paste this list exactly as is into notepad. Save it into Apprentice's "Decks" folder and name it whatever you want, such as "starterdeck.txt". Then, change the extension to .dec, such as "starterdeck.dec". You can then modify the deck inside Apprentice or in Notepad.