Addon to modify item tooltips (classic)

Hi All,

I had an idea for an addon for classic where I could modify a gear tooltip to show the secondary stats granted by a primary stat, for example:

+30 Agility (X.YZ%) Crit (X.YZ%) Dodge

Something like this where you could look at a piece of gear and get a quick idea of what it’s worth.

I’ve done basic addon work before but I’d like a bit of guidance on what APIs I need to look at to get started, if this is even viable/possible.

Thanks