I can help you with this but not much more, I'm not an Osmand expert...
1. Locate desired destination point on map, either by searching or touching and holding location on map until little "location box" pops up. Touch the box and a menu pops up, select "Set as destination".
2. Repeat 1) for starting point, except select "Directions From". Box will popup, select "Display route" or "Start navigation". If you have not already downloaded a voice file, Osmand will prompt you to DL file. Otherwise voice prompt will start immediately if you choose "Start navigation".
Basically you define the destination first, then define starting point (default is to start from GPS current location).