It's quite hard to give general advice on TWW3 as all races and starting locations work a bit differently but here goes.

TL;DR:

• Capture and hold your starting province with a single army.
• Build 2 armies.
• Capture and hold 2 full provinces.
• Grow your stacks, keeping income between 1-2k.
• Each time you hit 3k+ income, start a new army, dropping you back to around 1k.
• Use 2 armies to take a new province.
• Try to keep 1 army per province following these rules.
  • Before moving forward, increase public order to green.

Like in all the Total War games, wars are won through economy. If you can afford another army, go for it, banked money is wasted potential.

Settlements form provinces, and you want full provinces. Holding broken provinces is discouraged because it's hard to build and maintain the public order. Low public order results in slow growth and possible rebellions.

Which provinces to take depends on victory condition and global state. Check what are your victory conditions and focus on moving your area towards that. Otherwise pick weak, non-allied factions around you that have a negative opinion about you.

You can also take an isolated settlement to both hinder the province owner economy and later trade the settlement for a non-aggression pact.

You usually end up managing 4 armies. Some like to have 5th as a flex defender. Long games can end you up with 10+ armies.

Defensive allies and structures can reduce needed armies. Horde races require more flex armies than normal races.

Combat

Frontline is only as strong as its weakest point. In fights, spread your frontline to counter the enemy line, then concentrate all your remaining troops and spells to the weakest point to push through. Continue pushing your death ball on enemy troops one by one.

Province Progression

1. Construct one growth building in each settlement.
2. Research growth technology first if available.
3. Enable growth commandment when you get the full province.
4. Construct one or two control buildings so your public order is going positive.
5. Level all settlements to tier 3 before going for tier 4 on the capital.
6. Level up the capital to the max.
7. Replace all the growth buildings and commandment with something more useful.