In my experience they are fairly simple and obvious tools which can work really well.
I would not attempt any straightening or flattening unless you have a real basket case that has been stored in a damp place and has warped. Look at the iron in-situ and make sure that you have an even depth of sharp iron showing, and that the iron protrudes a little beyond the width of the sole.
The nickers should not be sharpened on the outside - that would reduce the width. Sharpen very carefully on the inside, using a fine needle file or slipstone. Each nicker should have three arcs to it - the outside and inside should be shallow arcs, and the edge (where they meet) should form an arc in the vertical plane. So the cutting action is that of a curved edge running across the fibres. The nickers on this plane work well and have the shape I am trying to describe:
In use, run the plane alongside a wooden straightedge, temporarily nailed or clamped in place. Start with a pull backwards to sever the fibres for the first cut; after that just push in the normal way, taking full length shavings.