Don't really like or suggst you use a diamond "steel"
With sharpening, ther are two main rules to follow:
1) Keep your bevels or the two angles that meet at a point, consistant
2) The finer the grit you use, the longer your edge will last.
With a diamond "Steel" you don't have very much control over the angle, and the diamond is aggesive and will remove metal.
I would get a minimum of a 4000 grit stone. If the edge is badly nicked, super dull (edge rounded over) or the tip broken off, a 1000 grit to do the "Grunt work" to get things back in shape.
The regular "Steels" are just there for "orthodontics" The cutting edge of a knife is tissue thin, and with use this gets bent over, The grooves of the steel willl pull this curled over edge straight again. You can do this multiple times, but after a while, the tissue thin edge willt fatigue from bending back and forth and break off--leaving you with a dull knife.