About the author - Jan Zumwalt, earned his Aeronautical engineer degree from the University of Alaska - Anchorage. Jan has over 2000hrs as pilot in command and has been employed as maintenance supervisor for several small commuter flight services. He also served as shift maintenance supervisor for Continental Airlines. He has designed and assisted many successful kit aircraft.
Do not start taxi tests unless the aircraft has 1-1/2 hour fuel supply and is loaded (with ballast if required) in the mid-range of its center of gravity travel; if you don't follow these guidelines, you may find yourself in the air because you are not at gross (lower stall speed) and your error in the indicated air speed may be significant. Remember, you also are in ground effect.
During the taxi runs, check the brakes, the steering and get familiar with the noise level, seat belt fastening and release, control position (carburetor heat, fuel valve, trim, mixture, etc.) and the important instruments such as air speed and rpm, altimeter, engine temperatures and pressures. When you do taxi tests at speeds above 50 percent of the stall speed given by the designer, be prepared to (accidentally) lift off. If there is enough runway left, throttle back and put it down again. Do not push it down, though - it's always such a shame to have to repair a brand new aircraft!
Make some 10 taxi runs on the runway to get the feeling of when to abort take off with sufficient runway to stop with moderate use of the brakes. This helps familiarize you with initial acceleration, and a bit (only a bit) with how the controls feel (light, heavy, sensitive, sluggish). Always trim at the mid-range as you are only guessing where it should be set.
Okay, your engine runs well, brakes work, instruments seem okay, you handle the aircraft with confidence on the ground (if it's a taildragger and you have little conventional gear experience that will take some time getting used to. A taildragger is less stable on the ground than a tricycle gear plane). It is very important you feel comfortable and "at home" in the pilot's seat.