In figure 1 is this complete circuit. The Diodes D1-D4 are regtifier ACV from 24 volts of transformer is DCV of about 33 volts both positive and negative voltage. The capacitor C1 and C4 is voltage filter from the bridge diode to smooth. The R1,D5 and D6 are made the reference voltage of +1.25 volts to the LM337T to adjust to 0 volt.
The R3,D10 and D11 make voltage -1.25 volts to LM317T to it can adjust to 0 volt as well.
The D7,D8 and D12,D13 protects voltage backward voltage from output. Which may makes IC Which may cause damage to the IC. The C2 and C5 are connected to reduce noise signal that from adjustable the potentiometer (VR1,VR2) and makes voltage at output to smooth up.