I got you. While I am not sure what YouTube video you are using, deep diaphragmatic breathing to reduce stress is a very studied topic. This is a big thing for athletes because stress can also be due to arousal that occurs during hard training.

All you need is something to help you learn the breathing pace and then you are good to go. Some sports watches and smart watches can even use biofeedback to help guide you.

The basic concept is 6 breaths per minute with 5 second inhale and 5 second exhale. This should be continuous breathing with no pause holding your breath or pausing on exhale. You should be using your diaphragm. Some studies change up the number of breaths per minute, but overall it's pretty similar.

Some research to read: While this is a research white paper and designed to be related to sports performance, they have plenty of cited studies and information on how deep breathing exercises help reduce stress.

https://www.polar.com/en/img/static/whitepapers/pdf/polar-serene-white-paper.pdf

Below are a few more.

Article examining bunch of studies to show deep breathing can help insomnia

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6361823/

Study showing reduced anxiety and better sleep for nurses who were dealing with COVID outbreaks. This study had limitations with no control group and being somewhat self reported.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7724962/

Study about deep breathing helping reduce stress

https://journals.lww.com/jbisrir/fulltext/2019/09000/effectiveness_of_diaphragmatic_breathing_for.6.aspx

Each of the linked articles and studies have even more cited studies to review about the effect of deep breathing on stress and the asymptomatic nervous system. So plenty to read and explore.