There are plenty of sites where you can find out how to address the English nobility, but I’m finding it difficult to determine how they might sign their names to a letter.
I’m especially looking about a real person, Baroness Louise Lehzen. Would it be Baroness Lehzen? Would she use her given names (Johanna Clara Louise Lehzen)? She’s only listed as a Baroness of Hanover, and I’m not sure if that was correct. I’ve even seen her listed as von Lehzen, but that was in Germany, so probably doesn’t apply (I’m talking the English court).
Lehzen would presumably have followed German, not English, conventions with regard to signing her name; the title she had was, after all, a German title and not a UK title, and she herself was raised and educated in Germany.
German baronial titles descend to all legitimate descendants, with the result that there may be many people at the same time holding and using the same title. So, Baron Manfred von Richthofen was the son of Baron Albrecht von Richthofen, the brother of Baron Lothar von Richthofen, the cousin of Baron Wolfram von Richthofen, etc. A great deal of confusion would arise if they all identified themselves simply as “Baron von Richthofen”, so they would generally have signed, e.g. Manfred von Richthofen, except in a context where no confusion could arise (e.g. a personal letter to a recipient who would know immediately who was writing to him). If there was a recognised head of the family who not only has the title but also held the estates, etc, he might have signed “Richthofen”, but I don’t beieve that there was any convention or expectation that he would normally do so.
In short, I don’t think there is any special convention about signatures for German barons; they generally sign their names in the same way as anyone else.