Can you be a bit more specific on the actual how-to ?
Sure.
First, you need a new empty map. Unfortunately, MapView won't let you create an empty file, you have to go and copy some old .map file, then rename it to the new name.
For example take a Supply Ship map - it's called UFO_170.MAP. Rename the copied file to something else, like for example MYFIRSTUFO.MAP.
Then, make sure that your Images.dat contains all terrain you want to use (files in the TERRAIN folder). If it's
X-Com 1, put it in the section starting with ${ufoImg}:${ufo}\TERRAIN\. If it's
TFTD, put it in the section starting with ${tftdImg}:${tftd}\TERRAIN\.
Let's assume you are going to use tilesets: TERRAIN1, TERRAIN2 and TERRAIN3. (Each of them is contained in three different files of the same name: .mcd, .pck and .tab.) If they're not in the Images.dat, under the right section, add them:
TERRAIN1{ufoImg}
TERRAIN2{ufoImg}
TERRAIN3{ufoImg}
Then open MapEdit.dat. Let's say you want to make a new UFO (as opposed to a terrain). Find this line:
Tileset:UFO - Ships
type:1
rootpath:${varPath3}
rmpPath:${varPath4}
blankPath:${varPath5}
palette:ufo-battle
This starts the section for UFOs (and crafts too). Put your new entry here. It should look like this:
MYFIRSTUFO:TERRAIN1 TERRAIN2 TERRAIN3
Now start up MapEdit - your map is ready for work! Well of course it contains the Supply Ship, so you'll have to remove all the tiles.
These are the most basic basics I could think of. Try it, experiment a little, ask questions.