A THREAD FOR NEWCOMERS!

There's an issue with managing battles. The problem is that, in almost any scenario, I consistently commit too many troops. When engaging in battles with roughly equal forces, I lose about 80% of the time. However, if I activate auto-battle, everything works fine; the computer manages to pull through, even if it's just barely in my favor. I'd like to control the battles myself without suffering significant losses, at least to the level of the AI.
Is this problem solved only with experience, or are there any tips and tricks?