1. first, compile a new openbsd kernel that has rthreads support. i assume you know how to compile an openbsd kernel. just take a GENERIC kernel and add option RTHREADS, compile it and boot it.
2. now, compile librthread, it's in openbsd 4.3 sources, just not compiled by default: cd /usr/src/lib/librthread && make obj && make && make install. this gives you a /usr/lib/librthread.a.
3. get the inferno code using subversion: svn checkout http://inferno-os.googlecode.com/svn/trunk/ inferno-os. this guide assumes you are storing it in your $HOME.
4. apply this patch: files/inferno-openbsd43-p1.diff. it fixes a few things, notably compile emu with librthread, make the jit work on openbsd. it also modifies mkconfig to install in $HOME/inferno-os, you might want to change that.
5. now fix your PATH: PATH=$HOME/inferno-os/FreeBSD/386/bin:$PATH, perhaps preferrably in your .profile if you run inferno more than once.
6. compile mk: ./makemk.sh.
7. now compile all libraries, emu and dis-files: mk install. make sure your stack size limit is higher than the default 4MB (8MB suffices), change it in /etc/login.conf. while you are in login.conf, you might also want to change the maximum number of processes a user can fork: the default is much too low for inferno.
8. you should now have a $HOME/inferno-os/FreeBSD/386/bin/emu and also freshly compiled dis files!
9. to eliminate font warnings and have nicer fonts, download inferno.tgz or cdinstall.iso.gz from the vita nuova download page and extract the fonts using 9e. xxx should be more clear here.