1. Select items to pause
Select your ad account and under Scope select ad sets that the rule will pause. The rule will be applied to all active ad sets on the account if setup is as follows:
2. Set max spend
In the first condition edit the Spend value. Set it to whatever amount of money you can spend with no purchases. You can always edit it later if you change your mind.
3. Get notified
Set up Slack or email notifications that you'll receive if the rule pauses anything. You can turn that off later.
You're all set! Now set the rule live.