How do I use the Army Builder screen to build a good army?

The Army Builder is the screen where you compose an army. The right method, in order:

1. Choose your 3 dragons (Left Flank / Center / Right Flank) — not the troop type first.
2. Choose ONE troop type for the whole army — all 3 dragons carry it. (You don't assign a type per dragon: it's one type per army.)
3. Look at the affinity (green/neutral/red) shown next to each dragon for that type, and favor the greens.
4. Fill troops to the max.

Good to know: changing the troop type doesn't change the army's "power number" — only the siege number moves (because affinity boosts siege). You can prepare up to 5 armies and leave them set, ready to go.