Double de-clutching is a thing of the past now. It's basically the same as marching while you're sitting down...left right left right
For the first left foot movement you take it out of gear and then give the throttle a blip.
The second left foot movement you put it down a cog, then bang the loud pedal again.
Left right left right.

The art is in knowing how hard to hit the throttle in neutral.

To double de-clutch going up a cog is much the same but without the blip on the throttle in neutral.

It becomes natural after a while.

Many years ago I owned a K5 Austin truck. If you couldn't double de-clutch or reverse by mirrors you couldn't drive that thing. It was so heavy in the steering you couldn't steer it slowly backwards with only one arm on the wheel. And being a slower revving diesel you had to get that blip on the loud pedal exactly right when changing down. There were no second chances.