Examine the space where the two floors meet, and determine if the floors are the same height. If it isn't obvious, place a level over the transition. Measure the difference in height, if any, and use the appropriate threshold.
Measure the width of the doorway or room opening where the two floors come together, using a tape measure. Cut a piece of scrap wood to 1/16 inch less than that measurement with a hacksaw. Lay the scrap wood in place and adjust the measurements if the scrap wood seems too short or too long.
Cut the threshold to the correct measurement with a hacksaw.
Put the threshold in place. If you are using a T-molding, fit the bottom part of the T in between the two floors.
Drive screws through the center of the threshold and into the subfloor below, using a power drill.