http://www.howtoforge.com/mysql_mast...er_replication
dunno if directadmin allows such settings - basically it's looking for a quiet moment, copy over the MySQL files and lock tables on the future master, then setting up the replica.
Before finally launching it - I recommend running a Repair run on the Slave - sometimes files copied with Rsync seem to be crippled.
And - also when rsync'ing - don't forget checking the file permissions on the slave