Everything revolves around the use of the following two screens:
The "Mini Map":
...and the "Warcraft IIx Dialog":
To begin: Select a unit in the main screen. The minimap-screen
will now show the name of the unit you have selected. Click on
the left button in the minimap screen:
You can now see that this unit's name is also shown in the "Warcraft IIx Dialog".
If you want you can now make some changes here and there.
When you have finished, click on the right button on the minimap screen:
Now you have changed the properties of a unit. If you want, you can now go further and choose another unit. By always doing it in this order, it is guaranteed to go well. Remember that some things are simply not possible. For example: it makes no sense to select submarine for a land unit (such as an Ogre).
If you want a unit to shoot with a (ship's) cannon, then select a small or large cannon (next to "missile"). Set the number to the left of the cannon to 4 (for example). This number indicates the range of the chosen weapon. On the right of the screen of the "WIIx Dialog" under "Unit Flags" you select "Can bombard".
Another tip: Sometimes if a unit is upgraded, it becomes another unit (for example: a Knight upgrades to a Palladin). In that case, you must change the properties of this upgraded unit as well.
It is possible to change the properties of units that are not already on the map. This way you can - for example - give the Archers a cannon and have the Rangers use the same cannon as well. Remember that you have to set this up! If you only give the Archers a cannon, then the Rangers will not have it automatically
Last tip: You can change any unit (even if they are not placed on the map yet), but you can not add new units to the map. For that you need one of the other editors, like the one that comes with Warcraft II. However, bear in mind that the regular editor will undo those special unit properties that the regular editor can not set itself.