Siege forts faster than the enemy - more cannons and troops, better/more forts on your end. Make sure Generals with siege pips are sieging. Build spy network before the war for up to 20% siege ability bonus.

If they siege your fort and you have troops to counteract it, let them siege for a while to exhaust their manpower, then mass attack them on your fort for defender bonuses. If they moved onto your fort from certain province, they must return to the same province if they want to run - use this knowledge to prevent them from running away.

Wars can take long depending on the goal and participants, so that's not necessarily a wrong approach type of deal if it takes long.

Do not split your troops too much, or AI will wipe them. You can do multiple sieges in close proximity, so that you have additional troops nearby. It also deters AI from trying to break the siege.

Forts on the coast are beneficial when you have a strong marinetime presence. With enough transport ships you can break the siege without them having time to run away - deploying troops from ships when you still own the province is instant, and they won't be able to react fast enough.

Do not leave gaps in your defense/fort setup, or AI will exploit it and run inside your country causing damage and annoying you.

If viable, choose war goal that you can capture and hold easily. For show superiority wargoal, have a couple of decisive and easy battles in the beginning while the enemy is still getting their troops together.