I have dug around MFP Support and Community posts and created this FAQ to help fellow Apple Watch MFP users. I've added a few things I've discovered as well.

How do I install the MyFitnessPal Apple Watch app?

If the auto-install option is on, the MyFitnessPal app for Apple Watch will install automatically once you update the MyFitnessPal app on iPhone to version 5.14 or later. (Integration with Apple Watch requires iOS version 8.3 or later).
If auto-install is off, launch the Apple Watch app on your iPhone, and visit the “App Store” app to manually install the MFP app from your iPhone to the Apple Watch. Please note that it may take up to several minutes after updating the iPhone app for the MyFitnessPal app to appear in the list of available Watch apps

What does Apple Watch send to MFP?

When syncing with your watch, you will track your exercises with the workouts app. This should then show in the Activity app as well as in iPhone Health app. It is intended to sync with MyFitnessPal if there is a matching exercise name in our database. If there isn’t a match it should still sync but may show as other. If you are not seeing them make it over, please check the Sources tab in Health and confirm that MyFitnessPal has permission to read Steps, walking+running distance, and workouts.

At this time we are currently only reading the step data. Individual workouts sent from the watch via what transfers directly from HealthKit. We are not yet considering the active calories amount, also being measured by the watch.

In the meantime, you may notice that your total calorie burn showing on the watch does not match the total being reported to MyFitnessPal by the HealthKit. This is to be expected.
We appreciate your patience and understanding as we work out all the kinks. We will also do our best to update the following link as we have more information available:
https://myfitnesspal.desk.com/customer/portal/articles/1957681


How do I enable step tracking in MyFitnessPal using the Apple Watch?
First, launch the MyFitnessPal app on your Apple Watch. This will register your Apple Watch with the MyFitnessPal app on iPhone.

Second, launch the MyFitnessPal iPhone app, and visit More > Steps. Select “Apple Watch” as your step source. This choice will use data from both the Apple Watch and the M7/M8 chip in the iPhone through HealthKit to track your steps.

What can I do with the MyFitnessPal Apple Watch app?
- Use step data from the watch to adjust your calorie goal for the day
- Walk more to eat more!
- View your remaining calories for the day
- ​View your remaining nutrient amounts for the day
- View your steps and your step goal for the day

Known Issues
- Exercises tracked as “Other” via the watch will not currently sync to the MyFitnessPal app. “Other” is a new HealthKit category, and the MFP app is currently unaware of this label. We hope to support this category in the future.
- The Nutrients view in the Watch app does not match our App store screenshots. This will be fixed in an upcoming release: Carbs, Fat and Protein will be moved to the top of the view.
- If the Watch app is unresponsive communicating with the iPhone, exiting and re-launching the Watch app is usually enough to cause unsynced data to process.
- In some cases it may also be helpful to tap More > Sync at the bottom of the MyFitnessPal iPhone app.

If you are having issues with the Apple Watch integration, try a fresh reinstall of the MyFitnessPal app and re-link your Apple Watch and HealthKit to see if that will help. Once reinstalled, in MyFitnessPal tap More>Steps>Apple Watch and you should be taken to a page to turn some Categories On. Turn them on and then tap More>Settings>Sharing & Privacy>HealthKit Sharing and you’ll be taken to another page to turn On more Categories. This should re-link your Apple Watch and HealthKit.

Deleting Accidental Workouts

iPhone>Heath App>Sources>Your Named Watch Device>Workouts. Tap 'Edit' and the tap on the entry to be deleted.

Apple Watch Workout Screen Metrics

iPhone>Watch App>Workout>Workout View

There are two big options: multiple metric and single metric. Single metric will show you a single statistic at a time, and spinning the Digital Crown will let you see others. Multiple metric will let you see more than one, and sort them in the order you to see while working out.
You can customize which metrics you see for which workouts. For example, for an outdoor walk your default metrics are duration, active calories, heart rate and distance. But you can also add current pace, average pace and total calories. For outdoor cycle, it's a little different. Duration, current speed, heart rate and distance are the default metrics; you can add average speed, active calories and total calories if you are inclined.

"Hey Siri, Start 'NAME OF WORKOUT' "

Starting a workout session from Apple Watch with your voice using Siri is the easiest way to get up and go. You can activate Siri on Apple Watch by raising your wrist and saying “Hey Siri Start [Name of Workout]” when the display is on to activate it.

If you just tell Siri to start a specific workout without specifying a time, distance, or calorie goal, Apple Watch will just start that workout session with an open goal too.

Siri can start specific workouts like a timed outdoor run or a specific distance outdoor walk by saying

“Hey Siri, start a 30 minute outdoor run” or “Hey Siri, start a 2 mile outdoor walk.”

If you just tell Siri to start a specific workout without specifying a time, distance, or calorie goal, Apple Watch will just start that workout session with an open goal too.

Log your workouts to get full credit

Using Siri is one of the easiest ways to start a workout, but why should you even take that step? Apple Watch is an all-day activity tracker after all, right? Starting a specific workout actually keeps the heart rate sensor fired up so you get precise measurements for the duration of your activity, and specifying which activity you’re doing helps Apple Watch know how to measure your movements.

Apple Watch Exercises
Currently there are some exercises in the Apple Watch that do not have a match in MyFitnessPal's exercise database. Due to this, those exercises will not sync. We are working on updating our database so that these exercises will have a match and will sync in the future. Below you will be able to find which exercises will and will not sync from Apple Watch.

The following chart lists the default Apple Watch exercises. Default exercises will sync to MyFitnessPal, and do not need to first be renamed after logging an Other exercise.

Apple Watch Exercises Will it Sync? Syncs to MyFitnessPal as:
*Elliptical Yes Elliptical Trainer
*HIIT Yes N/A
*Indoor Cycle Yes Bicycling, <10mph, leisure (cycling, biking, bike riding)
*Indoor Run Yes Running (jogging), 5mph (12 min mile)
*Indoor Walk Yes Walking, 2.0 mph, slow pace
*Open Water Swim Yes Swimming laps, freestyle, light/moderate effort
*Outdoor Cycle Yes Bicycling, <10mph, leisure (cycling, biking, bike riding)
*Outdoor Run Yes Running (jogging), 5mph (12 min mile)
*Outdoor Walk Yes Walking, 2.0 mph, slow pace
*Other Yes Apple HealthKit Workout
*Pool Swim Yes Swimming laps, freestyle, light/moderate effort
*Rower Yes Rowing, stationary, moderate effort
*Stair Stepper Yes Stair-treadmill ergo-meter, general