The guitar body is cut and pre-sanded, but to prepare the guitar for staining, I wanted to sand it some more. I started very gently using a 180 grit sandpaper. The veneer at the surface is very thin and easily sanded away. I then proceeded using 240 and finally 320 grit paper.
After wiping body and neck down using a sponge and warm water to raise the wood fibers and then let it dry thoroughly, I sanded everything lightly using 320 and finally 400 grit sand paper. I repeated this cycle two more times and ended up with a smooth top and bottom surface. I didn't manage to smooth the curved sides of the guitar beyond a certain point, especially where the wood was cut across the grain. Also, small fissures appeared at the bottom edge of the guitar, perhaps because of the watering -- or the cheap build.
When trying to pre-assemble the guitar, I found that the neck didn't fit into the neck pocket. I had to sand the sides of the neck's bolt-on block quite considerably until I got a snug fit. The same happened when I test-fitted the pickups. I couldn't set the pickups until I had filed and sanded down the corners of the cavities.
There were also a few places where during manufacturing some wood glue was allowed to seep or drip onto the surface of the guitar, especially at the sides and the headstock. It soaked into the wood and could not be removed by sanding. I can already tell that the guitar is quite shoddily manufactured. If you want to build your own, perhaps you want to find a higher quality one, even if it's a bit more expensive.
Anyway, the guitar is now as ready for staining as it's ever going to be.