Turn on the water supply to the hand shower
Most hand showers have an on-off valve so the user can turn off the water supply to the shower unit.
Try out hand shower to check the water pressure
I would suggest that you try out the pressure on the handheld shower before sitting down on the toilet. Water pressure can vary significantly and it is useful to establish the pressure levels before you get a surprise when using it.
Position the spray unit
There is a divergence of opinion here. I imagine the build of the use will have a bearing on which option is chosen. Some choose to hold the handheld shower by placing the hands down between the legs at the front. Others choose to move slightly forward on the seat and reach down behind to hold the unit. The idea is to spray upwards so that the dirty water falls back into the toilet bowl.
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Gently squeeze the trigger to begin spraying, being aware of the pressure that you checked earlier. At this stage, you may need to reposition the unit to ensure the correct coverage. Remember the ideal angle allows the contaminated water to fall down into the bowl and not onto your hand or the spray. Some hand units have adjustable angled nozzles so that you can find the optimum position.
Checking the results
For that extra confidence, many people like to use a small piece of toilet paper to check if all residue has been removed.
The best option at this stage is to have a towel kept for this purpose and dry yourself thoroughly before you replace your clothes.
Turn off the water supply
It is a wise precaution to turn off the water supply valve after each use. This avoids the possibility of a flooded bathroom and wasted water. Obviously, if your bathroom has a drain built into the floor, then the possibility of leakage is less of a problem, but it is still a wise precaution.
Most people find that once they have started using water to clean themselves that they will never return to their old ways. Learning how to use a western toilet with hand shower will result in a much fresher feel and that added confidence.