Usually the UK and US need to spend the first 2-3 turns clearing out Afr, so it is to Germany’s advantage to trade unit for unit with Russia, before the Allied hordes arrive. Weaken Russia so Japan can hopefully finish them off.
As for the initial question there are a couple ways to go.
First the standard EE stack which allows trading of kar/ukr/bel, but you should look to move a decent stack to Bel within the first few turns (2-4). This prevents Russia from continuing to trade Belo and keeps your stack safe from any Russian inf in Cauc. From here you can look to stack Kar or Ukr (depends on game). If you go to Kar you can cut off Allied reinforcements to Russia, if you go to Ukr you’re obviously looking to take the IC at Cauc.
If you make the Belo move in Round 3-4, that means the Kar/Ukr move should probably come in round 5 or 6.
Buy infantry heavy early (maybe 1 rt/arm if you have the extra 1-2 ipc) so you have a steady supply when you do expand.
The Second and Third option, would be to bypass the Belo move and go directly to Kar or Ukr as early as possible and hold there for a few turns and build up, but again this really depends on the game playout and what the Allies are doing.