Are you looking for an extract or plan to use all natural ingredients?
Chai is a spice blend put into tea. Look up a recipe for chai spice blend and you'll know what spices and in what ratios you want in your cola.
The easiest way would be to find a cola extract you like, and make it up as per the instructions, but instead of using plain water in your recipe, steep and strain your spice blend in that water first, then use that for the cola recipe.
I did a chai rootbeer once that was fantastic, I hope you find what you're looking for.