Hey guys G here and let's just dive, now there are three types of commonly used skeleton kisses

Bonk
Lick
French kiss

Bonk is usually first kiss, or used amongst family members. Like you mom or dad would kiss your head before you go to bed so basically since we don't have lips we bonk our heads together to say "I love you" in a more respectful family way.

Lick usually used when your in a more serious relationship but he/she doesn't want to French kiss or your not that deep into a relationship yet

French kiss same as humans

If you shy or nervous about French kiss it's always fine to bonk heads with us even if you don't kiss us because it means we know you care.

Story about Bonking
So when my three little munchkins ((Snas Underfell and Papyrus)) where little they would always bonk heads with me before they go to school. Sans and Paps who run up to me, jump up into my arms simotamiously and both bonk there little skulls and then I'd put them down and they would run off.

Underfell even back then was a tsundere because he would walk up to me with this little grumpy face with a pout and I'd get down on my knees. He would do it so quickly and then stare at me if saying tell no one it was always so cute to watch him run off after his two bros. ...