There are three conditions that allow full called shots, picking a specific component to aim at. The easiest is, as MarkDey mentioned, the Precision Shot ability.
The other two conditions are the target has either been knocked down (due to stability damage or a destroyed leg) or has shut down due to excess heat. You can make builds to exploit either of those conditions, but Precision Shot is the easiest way.
The other option is to use attack angles to increase the odds of hitting the legs. This will help even with called shots. Attacks from the sides have a near equal chance of hitting the side, arm, or leg, with a small chance to hit the center torso.
Attacks from the front are most likely to hit the torsos with low odds to hit the limbs. Same with attacks to the back.
If you don't have the morale to reliably do Precision Shots, go for the sides to tear off a leg and side torso. Then when they have been knocked down by leg destruction, you can make called shots on the other side or leg. Watch the initiative, falling down does reduce initiative by one phase, but a prone mech will always try to stand back up at the earliest opportunity.