Did you have any in mind. Non historical but thinking of the classic trumpet that existed at the time, it was a horn with no valves, similar in function and build to a modern bugle.

Brass is easy to work and a horn such as that could be hand beaten and rolled, then brazed together with a metal such as silver. I would imagine some templates and forms could be used for each specific range, as it would only have a few notes it could hit within the musical scale without modern valves. You can also do minor tuning to the pitch and tone by reshaping the bell.

I have no clue what the level of quality was, but I would hazard to say that it greatly varied. Still, the instrument is simple enough that a skilled craftsman could produce a high quality example with care, time, and trial and error.

Presumably you're referring to natural horns? That is, brass instruments with no valves?

There's actually a recent post in /r/DIY where a guy made a classic trumpet