Essentially it should be the same as double de clutching - clutch down, into neutral, clutch up to let engine and gearbox speeds equalise*, clutch down, into high range, clutch up. I don't think I would try it above 10 mph though....
* people above have mentioned revving the engine at this stage. That is only really neccessary when you are going down gears. goung up gears (as in low range to high range) you shouldn't need to revv it, just wait till the revs die off slightly, and feel the gear lever in.
If you want some practice, you can come and try my series 2a for a while... that requires it on all the gears!
Mark