When your archers are close enough to enemy units, a little target-looking icon will appear in their commands circle, which you click on. Then, during the battle, the archery mini-game will come up, and you have to click on the bow string and drag it to get the right power, and move the mouse to the position you want for the angle. My archery was overall very poor, and I might have killed as many of my own guys as I killed of theirs (remember that you'll be shooting into the same row that the archers are in).

Thanks for pointing the game out. Even with my archers sucking, I was able to win the first battle in story mode. Hooray for the Vikings!