All trucks are different. Try to put just enough pressure on the stick to the gear you want to go into and rev the engine slowly up about 1500 - 2000 rpm. It should just fall in. You shouldn't need the clutch to downshift. It is all about matching the engine speed to the tranny. If your rpm's are close enough - just force it in. Everyone grinds gears when they learn. It is normal.
Do you have a friend with a CDL? If so, rent a truck, practice for an hour or so, then take the test in the same truck. You REALLY should consider getting your class A.