[Aezeal]

OK you open a text editor

you save the file (now or later) as yourmodname.dm in the mods folder in dominions directory (as Sombre said)
later you select the mod (ENABLE) it in the mod proferences thread

If you want to change a unit you need to know a unit number (I personally look it up in Edi's DB) but you can also find it in game.

Then you do as Edi said

#selectmonster [monster number]
[commands]
#end

if you want to change HP
they you put
#hp 100
on the commands place (if you want to give the nit 100 Hp that is
if you want to change cost
you put
#gcost 100
to make the unit cost 100 gold

if you want to do both you go

#selectmonster [monster number]
#gcost 100
#hp 100
#end

You need to make seperate entries for each unit (specific number) you want to change.

All the commands are in the doc files in the doc directory and it's usually pretty clear how to use them.