Start with your yearly goal
You are not trying to complete the whole goal in 90 days. Start with the goal you already have.
It is: What needs to happen over the next 90 days to move this goal forward?
Look at where you are now
Your 90-day result should start from reality. What has happened already?
Define your 90-day result
Decide what meaningful progress you want to have to show for these 90 days.
“Publish three blog posts” is a result you can point to. “Work on my writing” describes activity.
Make the result concrete
Choose the kind of evidence you expect to have at the end of the 90 days, then refine your result if needed.
Check it against real life
A useful 90-day result works with the capacity you actually have.