G1 is a cheap B80C800 bridge rectifier.
The 7824 is a 24V/1A voltage regulator. It looks like this:
1=IN, 2=GND, 3=OUT.
You may also use a 78L24, a 24V/100mA voltage regulator. It looks like this:
1=OUT, 2=GND, 3=IN.
The resistors devide the 24V output voltage; the voltage across R2 (and R1) will be 12V. The opamp buffers this 12V voltage. The maximum power dissipation of the opamp is about 600mW, so make sure the output current doesn't exceed 600mW/24V = 25mA.
This is the component layout:
After soldering every component in its place, connect a power cord to power pole connector J1, and make sure the output voltage of the power supply is correct.