In a perfect world the corner should be a true 90 degrees so you would cut a 45 degree miter on the end of each mating piece.
It the real world things are seldom a true 90 degree corner. You can get an angle finder that will measure the exact angle of the corner, then each miter is exactly 1/2 of the angle.
In order to cut moldings that finite one needs an adjustable miter box or a powered miter saw.
The second option it to cut the 45 degree angles with a simple miter box, Use a sandpaper block to fit the pieces as best you can and fill the gap with wood filler or caulking.
Cut the end of 2pcs of quarter round with 45 degree and 135 degree separately. Then the ends of the 2pcs of quarter round can be put together well at the corner. The gap for laminate flooring can't be too big or too big.