1. If the datum and projection is not supported by OruxMaps, use Img2Ozf or MapMerge to convert to WGS84 and UTM with correct zone (see UTM grid map below). Remember to use the same pixelscale as original map. This will create ozfx3 maps.
2. If the map is of ozfx3 type, which is not supported by OruxMapsDesktop, use DeMapper to create jpg map.
3. Now you have a map in a format and with projection supported by OruxMapsDesktop.

UTM Grid map