You should see the video with Tony in the cab 12!
It's a matter of practice - double declutching takes a while to get used to and easier in a Chev than a Ford. It's a matter of not changing down when you're still too fast... back off throttle, disengage clutch, shift to neutral, give it a rev as you engage clutch, then disengage and go to lower gear. You need to go to the lower gear fairly smartly as blipping the throttle synchronises the gears and if you hesitate they slow down again causing a crunch.
Once you get the hang of it you can try doing it without the clutch. Very satisfying when they just snick in beautifully.
Changing up is similar but without blipping the throttle, and you can take it a bit more slowly - keep at it - the gearbox is strong!