Hello Fergal I hope you’ll forgive me for saying if navigating forest roads is a challenge for you then the mountain tops are pushing your limits even further?
From Invercauld Bridge through to Glen Gelder is dead easy to follow with a map. When it comes to Lochnagar itself the high routes are well worn in and identifiable and fine riding. Up past Meikle Pap is bouldery portage. The track NW from Carn an t-Sagairt Mor is a good descent. I’ve actually never used the track from Loch Callater to get onto Lochnagar.
I’ve done it a few times past Meikle Pap and down Carn t-Sagairt Mor, but if you get a fine day and want to ignore my comments in the first paragraph the best way is the full White Mounth munros circuit from Loch Muick or Crathie.
https://www.strava.com/activities/2422870232
You can get free grid reference providing apps for your smartphone if you don’t have a gps device, which can service as a back up to using a map and compass.