Name: WHM breathing
Rating: 4.8 (1712 reviews)
Author: MobileDriveway

This is tracker app for breathing exercise, similar to the one used by Wim Hof (WHM).

This app is 100% functional and free, but please getting a paid version of the app to support the development (see the link below). 

!!! PLUS version of the app !!! There are already some new features implemented in the paid version of the app (see its description), and I'll also be more responsive to implementing new features requests there:
https://apps.garmin.com/apps/3543ef5f-a9ea-48ed-a0dd-925155e9c7b5

You can also make a small donation if the free version is enough for you:
https://www.paypal.com/cgi-bin/webscr?cmd=_s-xclick&hosted_button_id=WTVK2Z25DCML2&source=url

This app helps you to track your progress. It creates a Garmin Activity, that you can review later. The activity will have the usual Garmin merics:
- Heart rate graph (you must wear your watch during the exercise to take the readings).
- Stress level graph (might not be available on some watches).

In addition, the app will create custom metrics:
- Number of rounds in the session
- Number of breaths in the session
- Maximum breath hold time the session
- Maximum breath hold time at the second stage of holding (after inhaling). WHM recomends holding for 15 seconds at that stage, but if you want to hold longer it is timed too
- Graph with a stage of the exercise (0 - pause between sets, 1 - breathing, 2 - holding, 3 - holding part 2). It will help you to map each stage to heart and stress graphs and see how each stage affects those metrics. Try bringing your heart rate down! You can also monitor your heart rate during the exercise. Looks like Connect app on Android is fixed now and shows the custom graphs for Breathing activity. If it still doesn't work for you, you can see the graph in Garmin Connect online: https://connect.garmin.com/modern/activities

Instructions
1. The app starts with the screen shown on the 1st picture. You can adjust your breath rate there - the number shows seconds per 1 breath. 3 sec/breath is default. Press UP or DOWN buttons (or swipe up or down on the touch screen) to adjust the time with 0.1 seconds step. Use app settings on your phone to do more precise adjustments.
You can also adjust breath rate during the session by pressing UP and DOWN buttons (or swiping up and down on a touch screen). However, be aware that the app recalculates breaths count for the current round based on the new rate, so you will see the count quickly change.
At the bottom of the screen you will see the results from your previous saved session. Press ENTER button (or tap the touchscreen) to start the new session, or press BACK to exit the app. ENTER and BACK buttons behavior can be switched in settings.
2. When you start a new session, you will see a screen similar to the picture #2. Instructions for each step will be displayed in the middle of the screen. Press ENTER (or tap the touchscreen) to start the BREATHING stage. 
3. After you reach the number of breaths you want (they are displayed on the screen), press ENTER and hold your breath. You can also end breathing stage automatically by enabling setting number of breaths in the app settings. A timer will start. The watch will also give a short vibration every 30 sec.
4. When you are done holding, press ENTER again. Breath in and hold. The watch will give a short vibration every 15 sec.
5. Press ENTER when you are done holding part 2. You can also end holding part 2 automatically by enabling setting number of seconds in the app settings.
6. If you are ready for the next round, repeat from the step #2.
7. If you want to finish the session at any time, press BACK button (or swipe right on the touchscreen). You can save or discard your session in a pop-up menu.

You can access SETTINGS screen by pressing MENU button (actuall button depends on the watch model, check your manual) or swiping Left at any time. In settings you can choose white background or disable vibrations