Don't mix basic rev matching with the more complicated double clutching. Double clutching is being in one gear pressing the clutch to take it out of gear, releasing the clutch when the box is in nuetral, revving the engine to the required revs, then pressing the clutch again, and THEN putting it into gear. I have to do this on my old land rovers as they are not synchronised in first and second gear. You don't need to do it at all on a car with a synchro. The one thing that you can do is basic rev-matching, (press the clutch change gear, rev the engine to required revs, release clutch) this doesn't make it any faster, but it does pro-long the life of your synchro, and leads to smoother gear changes.
Once you are used to it it becomes second nature, it's good to do as I feel that it tunes you in to your engine more, especially if you don't have a rev counter (the landrovers don't even have a working speedo lol), as you learn to listen to what your engine is telling you.