I got a search request for "janken words". This is what they are:

最初はグー、じゃんけんポン！
saisho wa guu, janken pon!
"First comes rock, jan-ken pon!"

The rhythm goes like this, with a little swing:

SAI - sho wa | GUU - - - | JAN - ken - | PON!*

You are supposed to throw your hand, either rock, paper or scissors, as you say pon, which is just mimesis.(Note also that you are supposed to throw "rock" as you say guu in the beginning -- this helps you and your partner get the timing down.) If you throw the same hand as your opponent, then you both say:

あいこでしょ！
aiko desho (maybe)
"It's a draw, right?" or possibly "It's a draw, so sho!"
AI - ko de | SHO!

You throw your new hand down on the sho. If you threw the same hand again, you repeat the aiko desho part until somebody wins.

When you get really good at this, you can play a multiple-round, multiple-participant, repeated-draw version without stopping the rhythm:

SAI - sho wa | GUU - - - | JAN - ken - | PON! - - -
AI - ko de | SHO! - - - | AI - ko de | SHO! - - -
AI - ko de | SHO! - - - (etc.)

There are all kinds of regional variations, of course, but I think that around the Tokyo area that's pretty much how it goes down.< /p>

My students also play the あっち見てほい (atchi mite, hoi!) bonus round described here:

As a game between two people, it has another level - the initial winner says 'atchi mite hoi! (look there!)' and points up, down, left or right. If the other person looks in the same direction, they lose. It's more difficult than it seems.

And it is, too.

* Note that due to the moraic nature of Japanese, this can blur into something like "SA i sho wa | GU u - - | JA n ke n | PO n - -". I haven't actually got my recording apparatus out to check, but my intuition is that even the two-mora syllables (like sai) are compressed into a single forceful beat here.