1. Start by strafing left and jumping. As soon as you hit the jump key move your mouse to the left about 35 degrees and don't stop moving it until you are about to hit the ground.
2. Now quickly switch your fingers to the right before you hit the ground and jump at the exact moment you hit the ground. Now repeat to build up speed.
Or the easy way -
bind "MWHEELUP" "+jump"
and heres some other commands -
bind "MWHEELUP" "+attack"
bind "MWHEELDOWN" "+duck"
bind MWHEELUP "cl_righthand 0"
bind MWHEELDOWN "cl_righthand 1"
