Step 1: Learn your toilet's parts
The lever pulls up on the flapper.
The flapper opens letting tank water fill the bowl.
Tank water level lowers and the fill valve float drops letting fresh water into the tank.
Tank water empties the flapper falls back into place.
Water from the fill valve fills tank.
Water pressure holds flapper down and raises the fill valve float
Fill valve float stop water from filling tank.
Step 2: Modify the flushing action
The trick is to obtain a half flush when you press the handle and a full flush when you press and hold the handle.
To do this disconnect the flapper chain.
Press the handle to 2/3 to 3/4 it's travel position.
Attach the chain to the lever so that the flapper is raised about 3/4 of an inch.
You may end up with slightly different measurements.