I've done it when my little one when he was 7 months, and I'm still a little traumatized 😂 but I was not prepared!
So, my first tip would be is to get noise cancelling earphone things for the baby to put on when it gets loud during the reception. You can also get a shade thing for the pram if s/he falls asleep.
Have you started weaning yet? If yes, bring some appropriate snacks, and agree with your partner/another adult to take turns minding the baby during the meal so you can eat.
During ceremony, sit at the back and pop him on a boob (if you are BF) if they get fidgety.
Oh, and wear something patterned, so inevitable smears and splodges won't be as noticeable (I've read this tip somewhere on the internet before we went, and it was great! Camouflaged smeared banana reasonably well!)
On the day, just go with it and try to have fun ☺️