Flashes just like a regular firmware. The only specific cases where you have to additionally reset your settings to factory defaults are:
- If running an RT-N16 with a firmware version lower than 3.x
- If running an RT-N66U with a firmware version lower than 18.104.22.168.220 (unless already running Asuswrt-Merlin)
In both these cases, you should reset to factory defaults before, and after flashing.
But in all cases, you can flash directly through the web interface like a regular Asus firmware.
Asuswrt-Merlin: Customized firmware for Asus routers
Github: github.com/RMerl - Twitter: RMerlinDev
See the sticky post for more info.