[mkgmap-dev] Multilayer Map in MapSource?

Hello guys, I'm currently new to generating maps for my Garmin eTrex Legend HCx. What I did till now: I generated a map with one single layer with mkgmap and installed that into MapSource. Now I found out that there's also the possibility to create a map with multiple layers (contours, basemap and routing map). Does anyone has an instruction which commands and ids I have to use? I already tried some things but without success till now. Thanks in advance