This one is actually easier than it sounds, but you may need to be patient before you can find a suitable town which can be easily captured. If you’re not too confident in your ability to capture a well defended settlement, you should look for factions which have been weakened by years of war. You can then prowl around their territory and wait for other armies to inflict most of the damage to a settlement, slowly eating away at the garrison troops and fortifications. You can then choose to take the town on your own if the previous attacker loses interest, or you can wait until the attacker takes it, then try to take it from them if they’ve been weakened by their efforts. Just keep in mind that you’ll be risking open warfare with whoever you attack if you’re not already at war with them.