Sounds like a great topic. But if you are going to test the efficacy of an intervention, you'll have to come up with a design that includes pre- and post-intervention measures and a control group.
Always much stronger if you use a variety of related measurements. I would suggest using a valid self-reporting instrument as well as a convenient and quick physiological measurement tool. Maybe a portable finger or wrist BP instrument - would also measure pulse rate.
You could start with baselines for all participant, that include "stress free" as well as "stressed" (perceived & measured). Randomly assign each participant to either the test or control group. THEN, teach the test group a coping mechanism and make sure the participants are using it .... THEN duplicate the initial testing procedures for both the test and control groups.
The only other way I can think of is to shift into a qualitative research mode - and rely only on interviews & self-reporting... not as strong, but if the purpose is to learn research skills it should be sufficient.