The Descent phase of your flight contains three segments: Cruise Descent,
Initial Approach, and Final Approach. Here is the Cruise Descent segment.

Maintain your cruise speed while descending to the initial approach point.
Plan to end your cruise descent about 10 NM from the field and at an
altitude of 3000 ft above the ground. "Ten and three" are easy numbers to
remember. Also be aware that if you are making an ILS approach that you must
be below the Glide Slope to properly capture it.

But when do we start our descent? Plan a descent rate of 500 fpm for all
your descents unless circumstances dictate differently. So at 500 fpm eleven
minutes will elapse to descend from 5500 ft down to Provincetown's near
sea-level elevation. At 135 kts cruise, you will cover 25 NM in 11 minutes.
I usually subtract out about 3 NM from that estimated descent distance since
the Initial Approach and Final Approach segments are flown at slower speeds
and less ground will be covered.

So begin this descent at 22 NM from the field. It will take five minutes to
descend from 5500 ft down to 3000 ft.

Note that if you have a 10 kt headwind during your approach that your Ground
Speed will reduce to 125 kts, and in 11 minutes you will cover only 23 NM.
For that case, begin descent 20 NM from the field after subtracting out the
3 NM.

At 22 NM from the field reduce the Manifold Pressure to 20 in.—with the
throttles—but leave the props at 2050 RPM. Adjust the throttles as needed to
maintain your desired air speed as you descend.

Reduce your power settings before beginning the descent to keep your
airspeed under control.

Whenever flight conditions require a large reduction in power, reduce RPM as
well as manifold pressure. As a rule of thumb, remember that each 100 RPM
requires at least 1 inch Hg manifold pressure; for example, 18 inches Hg at
1800 RPM. Operation at high RPM and low manifold pressure should be kept to
a minimum.

Never exceed 2325 RPM during descent.