Grab this file: http://annex.retroarchive.org/86-DOS/scp86mon.zip

Edit the scp86mon.txt to replace the "86dos.imd" filename with the name of
the disk you want to boot.

Start simh using "altairz80 scp86mon.txt"

Hit enter a few times and the SCP monitor prompt will come up, then hit
"B" to boot.