1. at home on a nice even surface such as a table calibrate it in the settings menu.
2. set up your car, weight, etc
3. put somewhere in your car FLAT and facing directly forward ( i use center console ) place it on one of those pads that are for putting cell phones on your dash so it doesnt move at all
4. from a stand still press start when it says go it is ready and will start whenever you go
you do have to start from a stand still every time so it can tell when you start moving. every run is recording 1/4 of a mile, you may stop before a quarter mile if you would like and it will still give you the results.
what i have found is that if its calibrated very well that it will be about + or - .3 on the times section.. and horsepower i couldnt really tell you. all depends on how much drivetrain loss you have configured in and different dynos read different etc etc etc.