Let’s say you have a rectangular room with the door on a short wall. You decide to have the new planks running across the doorway instead of perpendicular to it. At this point, divide the length of the room by the plank width to see what kind of partial width plank you should start [and end] with, or if it’s a small enough number that you can just start with a full plank? I’d say yes.

Install the underlayment as directed, and start laying that laminate, plank by plank. Be sure to leave an expansion gap around the perimeter of the new flooring of 5 or 6mm, about 1/4″. Cover the gp with baseboard, or a ‘base shoe or quarter round’ moulding.