??? Download OSM, write/draw intended route on map, then dowload ma...

Question to the forum in the form of HELP PLS. : I’d like to download an OSM, write or draw an intended route/course on the map that I’d like to ride (bicycle), and then download my route and map to a Garmin (305 or Nuvi)… The intent obviously is to follow the route line on the GPS… (currently you have to ride the course first, and save it to see it for future rides).

Is there an app tool available that creates a route that then can be downloaded with the map…? TNX ALL!!!