http://vectormaps4locus.eu/content/how

Be sure to have Locus FREE or Locus PRO installed on your device ;)
Download your desired map
If the map is called something like "blablabla.osm.map", leave it as it is
If the map is called something like "blablabla.osm.map.zip", unzip it
Copy the map to your device into the folder called "/Locus/mapsVector/"
You can organize your maps in subfolders, Locus uses them anyway
Start Locus & enjoy