- first of all: check if your kernel supports ext3!
(most kernels since 2006 do that anyhow, so you should be fine)
- optional step: touch /forcefsck
(this will trigger a file system check, not such a bright idea on machines that are not KVM oder KVM over IP manageable, as something might come up during the fsck)
- tune2fs -j /dev/sda1
(this created the journal)
- edit /etc/fstab so that partition gets mounted as ext3
- reboot / remount