I have a pump with what looks like an identical head. Here's the instruction pdf that they provide. The instructions provided are as follows.

1. Remove the dust cap from the tire valve.
2. For Presta valves, unscrew locking nut at the end of the valve.
3. Wipe off any mud or dirt from the outside surface of the valve stem; this ensures the pump's Clever Head will work properly. Also, release some air from the tube by pressing the air release tip on the valve to clean its inner parts.
4. Press the Clever Head on to the Presta or Schrader valve stem as far as it will go and open the thumb lever to the locked position.
5. The pressure gauge has a red arrow on the bezel that you can rotate to the desired pressure for easier reading.
6. Infate your tire by lifting the pump handle up and pressing down. Do not over pressurize tire; see side wall of tire for maximum pressure value.
7. Use the pressure release valve to lower and adjust tire pressure.
8. Move the thumb lever to the unlocked position and remove the Clever Head from the valve stem.
9. For Presta valves, tighten the locking nut at the end of the valve.
10. Replace dust cap

For presta valves, I usually don't under to locking nut the entire way, as I've had problems with them bending and snapping. Only undoing it half way seems to prevent this for the most part. Also, After I've mounted the pump in step 4, I press the air release button. If you hear air coming out, then it ensures the pump head is seated properly.

I have a pump with what looks like an identical head. Here's the instruction pdf that they provide. The instructions provided are as follows.

1. Remove the dust cap from the tire valve.
2. For Presta valves, unscrew locking nut at the end of the valve.
3. Wipe off any mud or dirt from the outside surface of the valve stem; this ensures the pump's Clever Head will work properly. Also, release some air from the tube by pressing the air release tip on the valve to clean its inner parts.
4. Press the Clever Head on to the Presta or Schrader valve stem as far as it will go and open the thumb lever to the locked position.
5. The pressure gauge has a red arrow on the bezel that you can rotate to the desired pressure for easier reading.
6. Infate your tire by lifting the pump handle up and pressing down. Do not over pressurize tire; see side wall of tire for maximum pressure value.
7. Use the pressure release valve to lower and adjust tire pressure.
8. Move the thumb lever to the unlocked position and remove the Clever Head from the valve stem.
9. For Presta valves, tighten the locking nut at the end of the valve.
10. Replace dust cap

For presta valves, I usually don't undo the locking nut the entire way, as I've had problems with them bending and snapping. Only undoing it half way seems to prevent this for the most part. Also, After I've mounted the pump in step 4, I press the air release button. If you hear air coming out, then it ensures the pump head is seated properly.