How do I ask for a raise?

I recommend following the advice from this lesson in the negotiation course: https://www.jointaro.com/course/the-insiders-guide-to-negotiating-your-tech-salary-and-compensation/negotiating-your-current-salary/

Are you sure it's purely additive and there's no backfill coming? Because if you're working 8 hour days and she was working 8 hour days, you would now need to work 16 hour days. That's not feasible, no matter how much you're paid. When absorbing another engineer's scope, you generally need to deprioritize certain things.

Regardless, just becoming more productive overall is good: [Course] Maximize Your Productivity As A Software Engineer

Phrasing is essential here. You don't want to make this a transactional exchange where you are doing X more tasks, and therefore you're asking for more pay.

Software engineers are not (should not) be judged by how many tasks they complete, they are judged on the impact they're having to the business.

If you talk about how you're more valuable to the business, then you can ask how that aligns with career progress or level promotion. Then use that as a jumping off point for more compensation.