One of the best ways to perform this test is a steady state effort on an inclined treadmill or stair machine. This minimizes external variables and can produce a more accurate result. This can be done with either a hiking or a running gait on the treadmill. Runners should do this running at approximately a 2-3% grade. Those who haven't run much recently may need to hike initially in the training and should set the treadmill to 15% and use a power hiking gait.
NOTE: Try not to use the hand rails for support on any of these machines for the test. Heavily relying on the handrails adds a variable and can affect the test results.

Indoors HR Drift Test on a treadmill or stair machine:

Warm up gradually for at least 10-15 minutes, never exceeding a nose breathing pace.
During the warm-up, you can adjust the pace while nose breathing to establish a starting pace where your HR stabilizes in a range of plus or minus 2-3 beats around a target number for at least 3 minutes. If your HR continues to climb during these 3 minutes, the pace is too fast. Slow it down until your HR stabilizes for 3-4 minutes. It is essential to let your HR stabilize at a consistent pace before starting the test.

After you find that stable HR pace, DO NOT TOUCH THE SPEED OR INCLINE FOR THE REST OF THE TEST.

This is the most common mistake folks make. If, say, 10 minutes into the test, your HR is 10 beats higher than the start and continuing to climb, the starting pace/HR was too high, and you will need to redo the test. Slowing the treadmill speed down during the test invalidates the result. It is probably best to come back another day to do the test again at a slower pace.

If all is good, though, continue this test for 60 minutes. Some gyms limit treadmill time to 60 minute, so you may need to do all this fiddling around during the warm-up, stop the machine, start your watch over again, and record a new session so you can go for an hour without interruption.

For more explanation and for a breakdown about how to analyze the results via Training Peaks, please refer to the following article:

https://evokeendurance.com/our-latest-thinking-on-aerobic-assessment-for-the-mountain-athlete/

For the outdoor HR Drift Test it is best to use an extremely flat course. A 400-meter running track is best if you have access to one. A rolling or hilly course will skew the data and make getting an accurate measure of drift functionally impossible, even if it is a loop or out-and-back. This method requires an accurate measure of HR (chest strap), good GPS data for pacing and distance, and being able to sync the data to your Training Peaks Premium account.

Warm up gradually for at least 15 minutes, never exceeding a nose breathing pace.
During the warm-up, you can speed up or slow down while nose breathing to establish a starting pace where your HR stabilizes in a range of plus or minus 2-3 beats around a target number for at least 3 minutes. If your HR continues to climb during these 3 minutes, the pace is too fast. Slow it down until your HR stabilizes for 3-4 minutes. It is essential to let your HR stabilize at a consistent pace before starting the test.

Once you have a stable HR, run for 1 hour, maintaining that approximate HR for the whole time. It doesn't have to be perfect as Training Peaks accounts for changes in both the heart rate and pacing.

Use a Training Peaks Premium account to analyze your heart rate drift by clicking on the analyze button in the data displayed in the upper right corner. You will see Pa:Hr. That is the HR drift. If your data file includes your warmup, you can use the cursor to highlight only the 60 minutes of the data file you are interested in.

For more explanation and a breakdown about how to analyze the results via Training Peaks, please refer to the following article:

https://evokeendurance.com/our-latest-thinking-on-aerobic-assessment-for-the-mountain-athlete/