There is an article in the knowledge base, but here in short:
losetup -e twofish256 /dev/loop0 /dev/sda1
mkreiserfs /dev/loop0
mount -t reiserfs /dev/loop0 /media/cryptpart
/dev/loop0 /dev/sda1 /media/cryptpart reiserfs twofish256 defaults
View the discussion thread.