I use use diamond stones for flattening the backs of chisels and plane irons. I like to first test the back with fine grit diamond stone. By using finer grit on a flat surface I can test whether the back is flat. If there are any areas that are not abraded by the fine grit I know that the back isn't flat.
Once I’ve determined that the back isn't flat I move to a coarse diamond stone. I find the diamond stones cut fast and stay very flat. Once the surface is uniformly scratched with the coarse stone I move to the next finer stone and repeat until I'm at the extra fine diamond stone. At this point I’ll switch to my finest water stone for the final polish. If you don’t have a set of diamond stones I suggest you use sandpaper backed by float glass to flatten the back of the tool.
If after the initial test with the fine stone the back of the tool appears flat, I just proceed with the next finest grit until I get to my finest water stones.
You can purchase Diamond Stones at SharpeningSupplies.com