Once in the project you would like to create a rule for, click "customize" in the top right corner.

Rules can be selected from the recommended list either by scrolling or by selecting the other software you would like the rule to work with.

An example would be adding subtasks to each task that is added to the project.

Task is added to this project → Create subtasks

Notice the action trigger has already been chosen for you. You would add the subtasks into the list under "create Subtasks" on the right hand side and click "Create rule". This would add that rule only to the project where you created it.

app.asana.com

This guide provides step-by-step instructions on how to create a rule or custom rule in Asana. It explains how to navigate to the rules section, select recommended rules or create a custom rule, choose triggers and actions, and customize rule options.