In the show, Koromon is Tai's digimon companion
"in-training". Koromon is basically a talking head that blows
bubbles as an attack. Koromon digivolves into Agumon
(Rookie) in the first episode, and later digivolves into Greymon (Champion).
Tai has the crest of courage and this allows his
digimon companion to evolve to Metal Greymon - the Ultimate level and then into WarGreymon.
The templates were originally made for a CD
(which is 4.75 inches in diameter), but anything that's round and about the same size
works... Make sure you ask an adult before you start gluing things to CD's!!