For your purposes (education) you could discharge at C/10 (36/10=3.6 amps) at room temperature. Because the battery is AGM it is safe to discharge it to 20% without damaging it. So, here's what you could do:
1. Fully charge the battery
2. Connect a 3.6 amp load and begin timing and monitoring voltage
3. When voltage reaches 11.35v disconnect the load and note the time the load has been on the battery
4. Continue to monitor voltage
5. When you disconnect the load from the battery, the voltage will begin to rise
6. Multiply the time (in hours) by 3.6 to tell you how many amp hours you have extracted from the battery
7. When voltage stops rising, use that number to determine the battery's true state of charge
8. Use the numbers obtained in #6 and #7 to extrapolate the battery capacity at a discharge rate of .1C
Since you are doing this for educational purposes, you could repeat the experiment to see the effect different discharge rates have on capacity. Then you could repeat the process to test the effects of temperature.
Here is one resource:
https://sunonbattery.com/agm-battery-voltage-capacity/