The best way to get card power is either from last hits or kills. If you're laning vs 2 heroes just let them push the wave close to your tower where you can safely farm.

Depending on your elo most people just auto attack the wave and push it completely ignoring last hits, so if you learn to last hit well you will be a huge amount of card power ahead of everyone else in the game.

For example typically in my games where I play a ranger I average a 50-100 last hits over everyone else in the game which puts me far ahead in card power. Pair that with a couple of kills and you can be unstoppable.

People typically tend to be better in the higher brackets and know better than to just auto attack push the wave which not only makes them miss last hits but also puts them into position to be ganked.

The best method for laning is to always make sure the enemy has 1-2 more creeps in their wave than you do, that way they will push towards your side of the map and you can either freeze the wave near your tower or just last hit safely in your side of the map.

If your jungler notices your creep wave management he might even swing around for an easy gank!