Choose the type of transition strip you need based on the thickness and material of the adjacent floor. Use a reducer if the other floor is thicker than the Pergo you're installing and a T-molding if it's the same thickness. If you're transitioning to carpet, use a carpet end-cap.
Install a metal track to hold the transition strip in the middle of the doorway. It should be 1/4 inch away from the existing floor. Screw it to a plywood subfloor with wood screws. If you're installing on a concrete subfloor, glue the track with construction adhesive and wait overnight for the adhesive to set.
Draw a line 1/4 inch away from the track with a straightedge and pencil. Install the Pergo flooring in the doorway. Whether you install the boards parallel or perpendicular to the doorway, the edge of the floor should be on the line.
Measure the width of the doorway with a tape measure and cut the transition strip to that measurement with a circular saw. Allow a 1/4-inch gap at each end of the strip so it will fit without your having to bend it.
Snap the strip into the track to complete the installation.