Slide a scrap piece of laminate under the baseboard heater to see if the laminate will fit under it whole. If you don’t have enough clearance, you will need to notch the pieces around the baseboard heater. Otherwise, you can install the laminate under the heater, cutting the pieces according to the wall.
Install your laminate flooring as normal as close to the baseboard heater as you can without needing to make a cut. Take the tape measure and measure out from the edge of your last full piece of laminate over to the baseboard heater. Mark your piece with the pencil and carpenter's square according to whether you need to notch an L cut around the edge of the baseboard heater, or according to the wall if you can slide the piece under the heater.
Place the laminate flooring piece on the tray of your miter saw. Pull the trigger and gently ease the blade down into the piece of laminate to cut the board across your mark. For an L cut, cut one section of the L, then turn the piece around and cut the other part of the L to make the notch.
Install your cut piece. Place the glue along the slots from the other, previous rows of laminate, slot your cut piece against the rows and tap it into place with the rubber mallet. Repeat the marking and cutting process for each individual piece after that, until you have worked your way around the baseboard heater.