Yeah it s hard, at first you should try to time your dunk with the death of a minion. If you have help in your lane, ask your fellow to take down archers as low as possible, or without help try to it yourself. It need practice but if you reach more than 30 before level 10, it's a really good start.

Once you get level 10, hit every minion one time, then black pool and throw your orb, that way you should be able to kill em all and get stacks.

OR wait for your fellow to land an aoe so you blackpool dunk on minions.

I like this guide, and it's a fun style to play. But it's really hard to get stacks.