Easiest way is to blitz and hold the enemy's capital territory, which typically puts you right at 100% allowing you to cut them down in size pretty quick. Attacking a smaller nation allied to a larger nation and making them give you territory from the larger nation in the peace deal. And if your target nation is smaller than your total amount of armies, you can just park an army on every territory during truce time so they can't rebuild.

As a great example, I was able to use all 3 methods during my WC as the Papal States/Italy by attacking and 100% occupying Haiti, taking 100% War Score worth of territories from the United States, then leaving my army on Haiti itself. And once the truce ran out, I just moved off, redeclared, and reoccupied, repeating the cycle until I fully annexed the US by fighting only 1 battle.

Similarly, I was able to quickly grow early on by attacking the UK and the Netherlands while they were fighting elsewhere, allowing me to quickly steal overlordship of Canada, Australia, India, and the East Indies from them.

Thanks man! ill try to do these things!