Athlon's PCB
After removing Athlon's cover, which is just as easy or painful as removing the cover from SECC1 Pentium II CPUs, you'll find three selection areas on the backside of the PCB, and one area on the front side of the PCB, where the core chip is mounted to. Those areas consist of clusters of four slots for SMD-resistors and the programming works by letting those slots either open or bridging it with the correct resistor. Altogether there are sixteen of those slots.
The backside hosts the voltage-selection area and the main multiplier area right next to it. You'll also find the first half of the additional frequency IDs close to the internal connector.
On the front side is the second half of the additional frequency IDs located, again close to Athlon's internal connector.