One of the most useful functions of a Garmin device is the ability to create custom workouts that the watch can guide you through. Not all Garmins have this capability so you should review the specs of your device to see if your device supports this. I have used this function with a 910, 920, and 230. You can create a workout on the device directly but I have found that using Garmin Connect online makes it MUCH easier.
In Garmin Connect, open the Workouts menu item.
From there you will want to create a new workout. You can choose the type of workout. I mostly make run workouts but I have made a bike one.
Once in the new workout, you can choose the type of interval, the duration of the interval (I often use Lap Button Press for my warm up and cool downs just so I can start and stop the workout when I am ready), you can add more steps, or add repeats (much easier than adding them over and over again)
I have added a set of repeats that will be 6 X 0.25 miles with 2:00 min rest. If you select the “Add More…” function you can specify a goal pace or HR zone for the interval. Be sure to name your workout.
When you are done you must SAVE the workout and then you can send it to a device.
If you have more than one device you must choose which device to send it to.
Next time your device syncs, it SHOULD pick up the new workout. This can be a bit clunky at times. You may have better luck sending it to your device through your phone if you use Garmin Connect Mobile. Give yourself some time to play with this step.
Once it has synced to your device you should be able to find it in a Training menu under Workouts. This will vary by device as well. Once you locate the workout you just have to start it! If your last interval called for pressing the lap button to end it, be sure to do that to hear your finishing music!