Nowadays it's easier to drive stick-shift-cars. In the ancient past, when the gearboxes weren't synchronized, you had the make the "double-declutch" ("Zwischengas" in German) by choosing annother gear.
It works so:
1.pushing the clutch
2.put out the gear
3.let the clutch back
4.short push on the gas without a gear
5.pushing the clutch again
6.put the other gear in
7.let the clutch back.

Quite tricky it's with that truck:
http://imcdb.org/vehicle_125484-MAN-630-L2AE.html
It has the "wanker-gearshift". You have to lift the stick shift up and down for getting the next gears.