I drive modern trucks for a living. The best description I can think of for learning to shift is to unload the transmission gears when shifting. In modern cars, folks (including me) accelerate continuously, then as soon as you touch the clutch, release the accelerator completely. That doesn't work in a truck. Accelerate, then level off or lightly ease up on the accelerator when you begin to gently pull on the shifter. You will feel the point where the load on the gears eases enough to smoothly pull out of gear. Keep the rpms fairly steady and it should ease on into the next gear. Again, some trucks may like a little more or less rpm drop between, but you'll get the idea.
