My bathtub drain leaks to garage below. What to do? I want to fix it myself.

Is there an access panel on the other side of the wall from the tub drain? Or can you see what's going on from the garage?

Hello, you need to gain access to the drain assembly, so you can inspect the tub drain and see where exactly the leak is at.
Then assess the situation, if it is the strainer leaking then unscrew the strainer with a spud wrench and clean the area and apply cilicone and replase strainer, let it set overnight to give the cilicone time to cure. Try it and hopefuly that's all you will have to deal with. Goodluck ---zeke---