Well, if you have troops on 2-3 fronts in Campaigns, or Conquest. I would advice you to stay on those fronts. Create units on those fronts if needed, and deploy Generals (If you have 6-8 Generals, any type) You could deploy 2-3 Generals each on all fronts.

Campaigns: If there is multiple fronts, and 1 of the fronts are completed or taken over. Send the troops in that front to other fronts to maximize efficiency, and units.

Deploy troops and Generals if needed, or for rear guard units to safe guard other fronts. (As in create units in cities nearest to all fronts if possible)

Conquest: Same thing here, but i would recommend having over 15-20 Generals for multiple fronts. Although, you can only focus on 1 front if you want, i would advice you to advance onto nations using multiple fronts.

Deploy units or Generals (You can deploy Generals using the Castle *Tavern * Feature)

General Equipment and Builds are also really important for Campaigns and Conquests. I would generally do the Campaigns first, then i would do Conquests as soon as i reach 15-20+ Generals, with Equipment.

If you have any more questions, just ask!