First you have to go online and sign up to be a developer. (anyone can do this) afterwards, you do the following:

1. Tap Settings (bottom-right)
2. Select your connected Quest from the device list and connect to it
3. Tap More Settings which appears below your Quest in the device list
4. Tap Developer Mode
5. Tap the switch to enable developer mode
6. Exit Settings on the app & reboot your Quest using the right-side power button