Defective Yeti says: Do not start playing Bowman because then you will not stop playing Bowman.

He's right.
Really evil. Finally stopped at 2:40 this morning. And I can't even say you didn't warn me.
Yeah it took me a while too: shot myself three or four times trying to figure it out.
Put the cursor in the "sky," just below the upper right corner of the screen, then press down the left mouse button and drag the mouse down and to the left. The number at the top of the line you'll make is the angle the arrow will travel and the number at the bottom represents level of force you are using to pull back the bow. Then let go of mouse button to shoot and adjust your angle/force for the next try.