There are a couple of routes to take from Perth – via Great Eastern Hwy, then Great Northern Hwy to the Brand Hway until you get to Midlands Road. Turn right and this will take you to Mingenew.
The other way is more inland. Take the Great Eastern Hwy, the Great Northern Hwy to the Brand Hwy. Turn right at Midlands Road and go through Mingenew. Then take the Mingenew Mullewa Road-Mullewa Road-Mullewa Wubin Road and back on the Mingenew Mullewa Road. Once you hit Geraldton Mt Magnet Road, turn right into Mullewa.
Transwa offer a bus from Perth to Geraldton (via Mogumber) on Sundays and Fridays. Also Perth to Kalbari (via New Norcia) on Tuesday and Thursday.