If you're new to PMM, what's a good way to think about, contextualize, and approach a 30/60/90 plan if you've never done one before? Also, are there any templates/resources you'd recommend as a jumping-off point?

I like to start with the purpose of creating a 30/60/90 plan. I view the plan as a set goals to help me strategize my first three months in a new job. I use it to help maximize my work output and stay focused. That said, I have rarely completed any 30/60/90 plan perfectly. 

Working in startups means lots of change and course correcting. It also means you're going to learn new information and have to adapt to it. Therefore, I advise PMMs to create a lose outline rather than a follow a template line-by-line. 

Here's what I'm currently doing in my new role at AWS (reminder it's a big company).

1. 30 days
1. Meet as many people as possible. I met over 40 people in my first days. Some meetings where very fruitful others where one time meetings. But this allowed me to introduce myself, what I'm going to be working on, undestand their role, and how we will work together moving forward. 
2. Complete HR & IT stuff (payroll, setup computer, etc)
3. Get access to tools that I'll use everyday 
4. Set up 1:1 meetings with my manager and monthly skip-levels with appropiate stakeholders
5. Understand team goals and how I contribute to them
6. Ask lots of questions with fresh eyes
2. 60 days
1. Start to internalize how the teams operate and cooperate
2. Understand how the business generates revenue and the levers that can be pulled to influence it
3. Understand the different team's goals (how are teams measured and how is success defined)
4. Complete necessary internal trainings
5. Formulate and document gaps in current process
6. Ask to be added to standing meetings to get additional context
7. Keep meeting more team members
3. 90 days
1. Start working on a cross-functional project and/or take the lead a new one
2. Complete outstanding onboarding tasks
3. Keep meeting more team members