Sandpaper, drywall screen, cinder-block, flat concrete, sidewalk or flat part of your driveway all works (I found the sidewalk outside of my house to be kind of smooth though). I've heard drywall screen clogs less than sandpaper due to the holes for grit and water to go through. Sandpaper or drywall screen needs to be on a flat surface to flatten, piece of tile or glass are good known flat surface examples. If sandpaper, be sure to get wet/dry ones, because your stone will need to be in the condition that you sharpen in, wet.
You may need to lap the polishing side of your stone after flattening, 300(+/-50) grit sandpaper would do (or another stone or nagura).
There's also $20 stone flattener/fixers that you should look into. Low priced and good alternative to sandpaper/drywall screen because it adds up if you flatten a lot. But in the super long run, a diamond plate or a good big stone flattener brick are the most economical and quick. BTW, eventually stone flatteners also need to be flattened or corrected.
Murray Carter has a Youtube video on stone maintenance that has useful tips on stone utilization, so you can flatten less often if you can make use of high spots on your stone.