To perform a Double Slash, simply click the left mouse button once instead of holding the button down. This will make you slash once to the left, instead of slashing back and forth. Now, click again. Try to give the space between your clicks little delay to make your Double Slash more powerful and fast. With practice, you will be able to Double Slash continually. Make sure you can Double Slash for at least 30 seconds without breaking rhythm before going on to the next step.
Now that you can Double Slash in place, it is time to practice on a moving target! Try to slide alongside him while slashing. This will allow you much more power than simply firing your SMG at him from behind. Finish with a chop. Once you can do this maneuver easily, practice the same except in the air. It is entirely possible to insta-kill airborne Bombers with Saber. Just perform a few double slashes, then chop to finish him off. Make sure you stay above his Q skill (Air Supremacy), or your Saber will do little damage to him.
Quick Finishing move: One double slash + a Chop. This will get any light class to about half HP in less than a second. I have killed an Assassin in midair with this move (he was at about 1/3 HP with full shield) while he was moving at around 80 MPH. Eventually, you will be able to add extra Double Slashes onto this basic combo for a slower but much more powerful move.
Tips: 1: You do not need to use auto lock with Doped Saber in every situation. Sometimes, it will only slow you down. It also does not work in Haze Bomb vicinity, so practice without auto lock when possible.
2: Use Double Slashing + Forward + Space to move with great agility and decent speed. It is possible to dodge other Scout's saber attacks and even snipes when using this method.
3: Attack your enemies from behind. Especially Blaster (or he can nearly insta-kill you with Micro Munitions), Tank (he can Maul you to death), other Scouts (remember, Saber is a powerful weapon in any hands), and Assassin (if they have Ambush Blades equipped.)
Here is a video explaining double slash partly: https://www.youtube.com/watch?v=m3WWj9fCuOoHave fun!