if you want to build plan9front in foreign plan9 installation do this after checking out the repository:

cd /path/to/plan9front

# create empty directories (hg wont track them for us)
. sys/lib/rootstub

# bind stuff to /
. sys/lib/rootbind

# build everything
cd /sys/src
mk nuke
mk install
mk clean

# use new binaries (only needed on clean checkout, 
# rootbind will do it for us when binaries are 
# populated in working directory)
bind /$objtype/bin /bin
bind -a /rc/bin /bin

# build manpage indices
cd /sys/man
mk

# build cd kernel
cd /sys/src/9/pc
mk 'CONF=pcf' install
mk 'CONF=pccpuf' install
mk clean

# build the iso
cd /sys/lib/dist
mk /tmp/9front.iso

in case you want to make the iso from your 9front installation, you can just do this:

# we are already in a 9front environment, so no special
# namespace preparation is required for building. just
# mk install the usual way in /sys/src

# put your root filesystem into /n/src9
bind /root /n/src9

# put your hg repository there
bind -ac /dist/plan9front /n/src9

# build the iso
cd /sys/lib/dist
mk /tmp/9front.iso