Click the details arrow for the task you would like to convert to a Milestone.

Select the 3 ellipses in the upper right hand corner of the details bar. 

There are two places you can see when a task has been converted to a Milestone. First is above the task name in the details bar. The second is using the green diamond symbol next to the task name in list view.

app.asana.com

This guide provides step-by-step instructions on how to set milestones in Asana, a popular project management tool. It explains two options for creating milestones: converting a task into a milestone and creating milestones from the project overview.

How to Set a Milestone in Asana

Option 2- Create Milestones from Overview