Remove the floor trim from the perimeter of the room using a pry bar and hammer. Don’t break it as you remove it; save it instead.
Lay out a course of foam underlayment along the side of the floor where you want to start the flooring, which is usually the longest unobstructed edge of the floor. Cut the underlayment to fit using a razor knife.
Lay the first Bruce locking floorboard alongside the wall at one end of the floor. Set wall spacers alongside the board to hold it out from the wall by a half inch.
Connect a second board off the end of the first one, setting the ends together until they click in place. Lay additional boards end to end along the wall, with spacers between all of them.
Use a miter saw to cut the last board to fit at the end of the course, with a half-inch space left there.
Set the second course next to the first. Connect the boards along their long edges by holding the first board down and pressing the edge of the second board against it at a 45-degree angle. Press down until the boards snap into place, and then lower the second board flat. Arrange the boards of the second course so the ends don’t line up with the boards of the first course.
Proceed along the whole length of the floor. Cut the last board as needed to fit at the end.
Continue across the room, course by course, laying more underlayment as needed. Cover the whole floor.
Cut the boards of the last course on a table saw so they fit against the ending wall with a half-inch space left there.
Reinstall the trim using your hammer and trim nails. The trim will cover the spaces around the perimeter of the floor.