There are a variety of methods. We did sell the Edge Pro flattener but I didn't like it. Big, heavy and messy to use.
There are a bunch of options to flatten. My friend Rookie mentioned a couple and they are good. In addition there are a couple other ones you should consider:
These are a few others:
The DMT XXC is the best selling plate we sell for flattening. Even, flat and long lasting.http://www.chefknivestogo.com/dmtextracoarse.html
This is another diamond flattener that is a little smaller and much less money:http://www.chefknivestogo.com/140grdistflp.html
This is a smaller one than the other 2 but it's 2 sided and it works on the Edge Pro for reprofiliing.http://www.chefknivestogo.com/dmt2x6diplfo.html
The Atoma is also available here:http://www.chefknivestogo.com/at14dipl.html
We don't sell sandpaper but you can get it at a hardward store. Get 120 grit wet dry sandpaper. It is cheap up front but if you sharpen a lot you will be better off with a diamond plate.