Copy ihelp.lua to somewhere in your Lua module path.
To get started:
$ lua -e "help = require'ihelp'"
> help()
To customize it for your application:
$ lua -l ihelp
> ihelp"customize"
and do what it says.
Alternatively, copy the help source into your own application and
provide your own shorthelp string and longhelp table.
With an existing LDoc-compatible module, e.g. ml (Microlight):
$ lua -l ml -l ihelp
> ihelp(ml)
and take it from there.
The module used to be called help but it seems help.lua is a popular
filename. Project homepage