To start using anacron:

Create the file ${SYSCONFDIR}/anacrontab (see anacrontab(5) for
information about its format).

The following is a simple example which runs the standard
OpenBSD daily, weekly, and monthly scripts:

-----Cut
# ${SYSCONFDIR}/anacrontab example
SHELL=/bin/sh
PATH=${PREFIX}/sbin:${PREFIX}/bin:/sbin:/bin:/usr/sbin:/usr/bin
# format: period delay job-identifier command
1  5  cron.daily   sh /etc/daily 2>&1   | tee /var/log/daily.out
7  10 cron.weekly  sh /etc/weekly 2>&1  | tee /var/log/weekly.out
30 15 cron.monthly sh /etc/monthly 2>&1 | tee /var/log/monthly.out
-----Cut

Comment out the invocation of these jobs in root's crontab.

Invoke anacron from /etc/rc.local like this:

if [ X"${anacron}" == X"YES" -a -x ${PREFIX}/sbin/anacron ]; then
    echo -n ' anacron'
    ${PREFIX}/sbin/anacron -s
fi

(and add "anacron=YES" to /etc/rc.conf.local)

If your machine is left running for more than 24h at a time, you
might also want to invoke anacron from an early morning cron job.