The rules from the Primary School Slang Handbook state:
"One must be skill with two 'L's and not just one. This has to be clearly stated after the word is said. If a person is 'Skill' with only one 'L' it means they have horses bum disease"
Ray - "You never said skill with two L's, that means you have horses bum disease"
Normally used by holding both thumbs up (a la The Fonz in Happy Days) and saying "Skill!" with a big cheesey grin.
"I've just cleared the 128th screen on Pacman! Skill!"