My offer letter mentioned a $10K bonus if I did really well. How can I get the bonus?

Ideally you want to get clarity on this kind of bonus (which is substantial in this case) in writing, before you join the company. The main mission piece now is that “really well” is not measurable.

What are the quantifiable ways to check if you’re doing really well?

• Typically at more junior levels, these goals will be more input-driven: building a major feature, writing a certain amount of code, or finishing a migration.
• At more senior levels, the metric is usually more output-driven: increasing weekly active users by some number, or reducing churn, or improving the app startup time.

One thing you can try is to present a plan to your manager about various outcomes of your work in X months, and provide your own definition of what ok/good/great look like. Then ask your manager for input, and use that as a starting point to determine where you’d both feel good about the $10K bonus getting paid out.

Yeah, this is a very "fluffy" type of agreement, and it's going to be very hard to enforce. These kinds of scenario often led to bad outcomes as party X believes they deserve the reward while party Y doesn't, and there's no objective way to hash out the disagreement.

When it comes to tech in general, think in terms of contracts. Whenever there's anything important (it can be this performance bonus, a deadline for delivering a dependency, etc), try to get things in writing to show that there's an explicit agreement between the relevant parties. This makes outcomes clear and fair. For this bonus, figure out this contract with your manager and maybe HR as well.