For all gear changes, use the clutch twice.  For example, first press the clutch down fully, then shift from first to neutral, release the clutch; second press the clutch down fully again, then shift from neutral to second.

When shacman parts shifting from low gear to high gear, for example from 6th to 7th, first shift the valve on lever from "L" to "H", then step on the clutch, move the gear to neutral, release the clutch, then step on the clutch again, move the gear to 7th. (Shacman Truck Parts)