Difficult question, some people etch with vinegar like Teodor describe, others use only Ferrichlorid but there are various other etching agents. Most of the time collectors swear by their technic as only good one.
I think that different steel react better with one or the other. And it depend on the wished result.
I personally etch as well with vinegar similar as described in up but heat the etchant and blade, the reaction is faster and deeper.
Of course you get better results when the blade is fine polished and free from rust so far possible.
Regards,
Detlef