Simplest possible setup (FreeBSD)

/etc/rc.conf

pf_enable="YES"                 # Enable PF (load module if required)
pf_rules="/etc/pf.conf"         # rules definition file for pf
pf_flags=""                     # additional flags for pfctl startup
pflog_enable="YES"              # start pflogd(8)
pflog_logfile="/var/log/pflog"  # where pflogd should store the logfile
pflog_flags=""                  # additional flags for pflogd startup

Fortunately most of these are the defaults, you need only add

pf_enable="YES"                 # Enable PF (load module if required)
pflog_enable="YES"              # start pflogd(8)

enable with

FreeBSD:~$ sudo pfctl -ef /etc/pf.conf

or

$ sudo /etc/rc.d/pf start