All you need to know is in ssuser's BETTER Battlezone 1 Map Editor Guide, which I believe is both on a forum of this site, as well as available as a download (maybe not here?) but at many Battlezone 1 websites.
Starting from scratch takes a bit more work, (don't worry, not that much more) but the documentation is all there in the editor guides on how to create your own map.
But say you wanted to edit a single player mission, say the third mission.
They way I do it is:
1) Create a shortcut to bzone.exe and place it on your desktop.
2) Right click it, select properties
3) Where it says "Target:" then "blah blah blah blah\Battlezone 1.5\bzone.exe" , add misn03.bzn /edit /win /nointro /sw
(with 1.5, adding /edit alone should be fine)
4) Accept, double click your shortcut.
5) Press control + a to enter object placing mode
Press control + e to enter terrain painting mode
once in control + e mode, press w to enter terrain editing mode
MY shortcut has this "Target:"
"C:\Program Files (x86)\Games\Activision\Battlezone 1998\Battlezone 1.5\bzone.exe" misn03.bzn /nointro /edit
If you want to edit in Battlezone 1, you NEED to head over to Ssuser's guide here:
http://www.battlezone1.org/ia-mission-tutorial-t1135.html