I copied this from multiple pages of this site, I hope it is clear anyway:
Set treadmill to 10% and begin hiking slowly. If training for flatter runs, set treadmill to 3% and run. Gradually build speed over the first 10–15 minutes until your heart rate stabilizes at what you FEEL is an easy aerobic effort. If you have a good idea of what your AeT is, then target that heart rate for the beginning of the test. NOW YOU ARE READY TO BEGIN THE TEST.
NOTE: If hiking, you may need to use a steeper grade (10%+) in order to get your heart rate up sufficiently.
VERY IMPORTANT: Once that speed and grade are dialed in, do not adjust them again during the test. Run or hike continuously for 60 minutes at this speed. Record your heart rate and upload it to TrainingPeaks.
* Stay on the treadmill for at least 30?-60? (the longer, the more reliable the test);
* Record the workout, pressing the lap button halfway through;
* When you’re done, divide the average heart rate of the second half by the average heart rate of the first;
* Lastly, subtract one and multiply by 100 to get the percentage HR drift.
For example, if (2nd_half_HR) / (1st_half_HR) = 1.091, then the drift was 9.1%.
AeT is where the drift is at about 5 %.