The circuit is simple. The 12v source runs through the two SPDT buttons so that, when the buttons are not pressed both leads of the motor are 'shorted' to ground. This gives you electro-mechanical braking.
One of each of the 2nd taps on the SPDT buttons connects to the positive side of the 12v source. This source has a 200R resistor (change as necessary for your motor, but 200R should work), a 470uF electrolytic capacitor in parallel with the resistor and an on/off switch to 'short' the 12v positive AROUND the resistor/capacitor pair.
The resistor provides a current limit for slow speed and the capacitor provides a momentary 'kick start' to get the motor running. With my motor the current is not enough to start the motor but is well enough to keep it turning. The only caveat here is to run the focuser back and forth once in high speed for a second to allow the capacitor to charge. Once it's charged, it will work at low speed all the time.
The top graphic shows a 2-speed controller with high and low, the lower graphic shows a variable speed controller with the switch and resistor replaced by a suitable potentiometer. It works the same.
With my Conrad DC motor and 132:1 gearhead high speed turns the output shaft at 60RPM and the low speed at 12rpm. It works great.
The 12v input should be captive to the unit with enough wire to make it to your power source. The motors can be connected using any type of plug you desire.
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