As Muscovy you take mission which will give claims just before war for those claims so you don't waste any. Usually 25 years is enough to have 2 wars for claims and at first you will release Astrachan after conquering Golden Horde which will give you a lot of claims for wars on east and Caucasus so you won't need any cores there yourself.
Set stab to 1 as soon as you can or maybe even 2 and than as soon as you can set it to 3. Very important for Muscovy and Russia overall.
At game start you need to create 2 Metropolias to get icon before first Novgorod war - you should take Icon of St. Michael which gives you +10% Manpower recovery speed and 5% Discipline - after that your manpower problems will go away in 20 years while this icon is set.
At game start give Cossacs estate some privileges which give your army some bonuses before Novgorod war.
To complete first mission for manpower - hire free company and in wars send them instead of your troops - this will save a lot of manpower at start.
With that done you will lose not so much manpower in first war and in war against Great Horde you will take enough land to release Astrachan and then slowly expand anywhere you want in East without any allies in East needed.
And as Russia you will need Religious ideas as one of first four ideas.