Kissing can be an art in itself. There are a number of people in this world that can rock your world with only a kiss. Unfortunately there are also many people in this world that, well, they suck at kissing.
How do you tell someone that they kiss like a fish/your grand mother/a nun/your dog fido/a blow up doll/a maniquin/a barbie doll/a slice of liver/a sponge/a bowl of raw calamari...etc...
You don't! you just don't kiss them EVER AGAIN.(luckily there are people in this world who have the grace and understanding to put up with some bad kissing long enough to help the guilty party overcome their ignorance, but there are limits!)