You should start by finding a reliable pressure gauge or a pump with a pressure gauge.

You should start by finding a reliable pressure gauge
or a pump with a pressure gauge. Then, use this same
gauge or pump anytime you are making adjustments. A
gauge can be very inaccurate, so if you switch around
it you can make things much more difficult.

You should start with a higher pressure of around 40 -
50 psi. If you have a tubeless system, you should
start lower, 30 - 40 psi. The more you weigh, the
higher pressure you should start with. Try this
pressure for a while and get a feel for how the tires
take corners and loose dirt.

Drop the pressure by 5 psi in each tire and get a feel
for how this new setup rides and how it compares to your
previous setting. You should notice some improvement
in stability, and if you don't, drop the pressure by
another 5 psi.

You want to find the lowest pressure you can ride with
without sacrificing pinch flat resistance. A pinch flat
occurs when your tire rolls over an object then compresses
to the point where the tire and the tube get pinched
between the object and the rim on the wheel.

With tubeless tire systems, you can run much lower air
pressure, as you don't have to worry about getting pinch
flats. If you start to dent your rims, burp air out
along the bead, or feel the tire roll under the rim
during hard cornering, you've taken the pressure much
too low.

Once you've found a comfortable setting for your tire
pressure, learn what your tire feels like when you
squeeze it with your hands. Once you know what your
tires feel like you can always get the right air
pressure - with any pump.