Not to disappoint, but AFAIK there's no mandatory baptism ritual (unless Bobby's book has specified one). If you feel that a baptism is important, go with what feels right. In this case, a dab of pasta sauce will probably suffice.
We also prefer to minimize sectarianism, at least around here. Flying Spaghetti Monsterism is built from the individual upwards, not from a leader downwards, so any regional differences in worship and rituals are blessed in the eyes of His Noodliness, provided nobody gets too up in arms about their way being the RIGHT way. I've got a nice preserved black bean sauce recipe that could work as well as marinara, or if you're thinking total immersion (might be practical for Atreau), how about soba sauce instead (nice and cool for those hot days... it's been soba for dinner the last couple of nights at my place)?
Looking back, my 'baptism' took place before I heard of the FSM, when I dumped a pot of near-boiling pasta water all over myself (worked as a cook in a pasta place for 5 years).
Hope that helps!