generally speaking when you are cruising along and a long hill is slowing you down, you move the button on the gearshift from high to low range, then while keeping the gas pedal down push in and out the clutch pedal real fast. To go from low range to high range move button and just ease off the gas and gently reapply power.
A few notes , the speedometer will not show true speed in low range. Also you need to find out how fast you can go in a given gear. Example 5th gear low range might give you 45mph. So if you are in 5th high range and doing 60 then do not downshift until you get down to 45.
On my bus the shifting instructions are riveted to the glovebox lid.