Sand really well and clean with acetone. Lay the mats down first to be sure they are where you want them. If not using the 3M adhesive and you are using contact cement, use three light coats on each side, letting each dry completely before applying the next. A second set of hands is helpful to get them into position. Start at one end and gently lay them down starting at one end and working back. So the sides first, and the tray last. Beat the mat with a mallet to get the air out.
My tip of the day is to use dowels between the peices when aligning. Keeps the two from sticking together before you get it where you want it.