Well this thread probably has some of the worst information in it I've seen in a while, so let me clear it up.
DON'T use the sidewalk on these stones. Massive grit contamination of the stone and these stones would be rough on your sidewalk. BAD idea for these stones and a BAD idea in general for most any stone. Using the sidewalk on stones sucks as an idea - one of my pet peeves.
If you have a crappy diamond plate it will eat it up so don't do that. If you get a diamond plate the DMT plate will work OK - not recommended. The Atoma 140 will work for minor dishing and holds up much better. BUT the result will be that the diamond plate will flatten THROUGH the 24 or 60 grit particles. so you will get a much finer finish than the stone is capable of. You want a rough surface for these stones to do what they are intended to do.
You can flatten and properly texturize the 60 grit with the 24 grit. Works great!
The 24 grit stone dishes slowly BUT it will eventually dish. So what will work on it? It will eat a drywall screen for lunch - bad idea. Sidewalk - another bad idea (already mentioned but why not kill this idea twice
) Sanding belt glued to something - it will eat a 24 grit Blaze belt too - only a moderately bad idea.
If you want to get this stone truly flat you need two of these stones. This is true of other stone flatteners too - Nortons, Naniwas, pink ones, etc. And you can use this stone to flatten all of them because they will dish even more easily than the 24 grit Nubatama. This one is as rough as it gets for stones.
You can use some silicon carbide particles on it - the coarser the better, but it's messy. I've done it with the large Naniwa flattener. I don't use this technique anymore. If you are using finer loose grit, use a finer stone too.
THE classic way to flatten stones - long before diamond plates existed was to use three stones. Flatten A against B, B against C and C against A. This is ideal but you can get by with two stones just reversing the direction of the stones.
If you are a serious sharpener, I highly recommend these stones. For lighter work the 120 or 150 Nubatamas are fine - and easily flatten against the 24 grit stone too.
As an aside if you have a buffing wheel with very coarse compound (like 120 greaseless) and need to true your buffing wheel, stuffing the 24 grit stone on end into the wheel will rip the compound off - and generate a nice spark trail. Otherwise you just eat up your rakes. For repairing chips and tips, this is an excellent stone - either the 60 or 24 grit.
It does leave a coarse finish, so expect to clean it up with finer stones or even the 140 Atoma. Going from the 60 grit Nubatama to the 150 bamboo is very reasonable too.
Once you learn to use these stones correctly, they are quite useful.