Place your hardwood laminate board on the ground next to the door frame's architrave. Make sure that you've already installed the underlayment for the flooring. If you haven't, the measurements you take will be wrong.
Push your hardwood laminate board against the bottom of your then frame. Make a mark with a pen all the way around the architrave where the top of the board meets it.
Remove the part of the architrave below the mark you made with a handsaw. This will allow you to slot your hardwood laminate board beneath the architrave.