1. Jump forward
2. Look ~30degrees left of your current movement direction, hold left+forward. This will cause you to accelerate to left and a bit forward, even past max velocity
3. Hold jump every time before you hit the ground and release right after to jump instantly on landing and avoid friction
4. Switch both aim and strafe direction every jump to avoid drifting off course
5. The required angle approaches 45 degrees as your velocity increases. Higher velocities require more precise aim. Aiming too far to the side (of your movement direction) will cause you to slow down
straftrain1 is best map for practice
use /cg_drawSpeed 7
(What matters is accelerating ~75-90 degrees to the side of the current movement direction, which can be done with left+fwd + aiming ~30 degrees to the left, or left + aim ~15 degrees to right, or with some other combination, but the 1 described above is easiest IMO and has less risk of accidentally slowing you down.)