I know the world of computing has forsworn manuals
for a variety of reasons, but it's a valid approach in
Plan 9.

So, read uart(3) first. It explains how to configure
a uart by opening the ctl file and writing simple
strings. You read and write it by reading and writing the
data file. As they used to say on a game show here,
"that's all there is to it".

If you need more, then try

src con

the source of the terminal emulator.