How do I install bathroom sink knobs?

Are you talking about a faucet handle? Not seeing what exactly broke, kind of hard to help.you. May be easier to replace the entire faucet.

A photo might help us to help you.

Yep a photo would help. The broken knob and what it broke off.

Hi Eve,

Please send a picture so you can get the fix it help you need.

Unless you purchased the exact same sink faucet handle, size and threads may be different.

Hi Eve! Is a piece still in the fitting? Usually, when I have trouble replacing a part, either the fitting is corroded with build up or something has broken off from the original part. Adding a picture would help us suggest more.

Here is the picture