Hello Kiko!
There are 2 type of maps to edit.
1. Heroes 5.5 map - Open it with editor do the changes and save the map. You will get warnings about "adventure map objects spells not set". Disregard it. This is due to spells and rewards being randomized.
2. normal heroes 5 map - Old maps need to have their scripts activated so they can offer all the benefits of the mod.
Check
this guide it shows how to create a new map and activate scripts.
When you have created a map, put it in
/maps folder.