Nanning is a city in China and the capitol of Guangxi Province.
From the Guangxi Daxue (University) Busstop take the Bus Number 76 all the way to the last stop Langdong Bus Station 琅东汽车站 or take a metro to Langdong Bus Station. Walk to the right side after you leave the building and cross the junction, then you should already see the toll station. From there it is super easy to find someone going that direction. User Ruebezahl got even approached and asked if he want to come with the driver to Liuzhou, before he could ask anyone.
UPDATE: SEPTEMBER 2019 The toll station is no longer there but this place is still the place to hitchhike from as it is forbidden and impractical to hitchhike further down the highway. Use a (big) sign to attract the attention of the drivers on the 4-lane highway.
South-West towards Vietnam border.
From the center of the city, take bus number 5 to the bus stop just after you turn onto Nahong Avenue. After getting off the bus, walk south along the motor way for about 20 min, passing through a little forest before emerging at the gas station and toll gates at GPS location 22.748040,108.271346. We had a ride in 10 minutes with a sign at the exit of the gas station.
Once you get to the border, be sure to change your Chinese yuan for Vietnamese dong at the shops on the Chinese side. It would be hard to find ATM with dong until Hanoi.