Summarize this article:

Release the blade lever and move the base plate that surrounds the saw until 2 inches (5.1 cm) of the saw is exposed. Once the blade is the right depth, tuck your finger under the shoe with your index finger to hold it steady and tighten the blade lever.  Always unplug the circular saw before adjusting blade depth. If you're having trouble setting the right depth, find a long piece of wood 2 inches (5.1 cm) thick and use it as a guide. to a 15-amp circular saw. Attach a pair of vice grips to the edge of the old blade so that it can't move. Use a wrench to remove the center bolt holding the blade in place. Afterward, remove the small rim (also known as the flange) underneath it and take out the old blade. Attach a diamond blade so that the teeth point in the opposite direction of movement. Reattach the flange and retighten the bolt.  If the diamond blade is moving around while you're screwing it on, attach vice grips to it. Use dry-cutting diamond blades if you're making a series of cuts that increase in depth. Invest in wet-cutting diamond blades if you want to make fast, clean cuts and you have a compatible saw. Do not over tighten the blade-locking nut. Get on your hands and knees and face the chalk line at a 45-degree angle. Grip the front-most handle of the blade with your dominant hand and use it to guide the saw. Use your dominant hand to grip the back handle and push the blade forward and backward. Begin cutting at the edge of the concrete and slowly follow the chalk line.  Don't press the blade into the ground—let the weight of the blade and saw do the work. Remove the blade every 30 to 45 seconds to avoid overheating. If you're not using a wet-cutting diamond blade and the saw is gas powered, pour water onto your blade as you cut to reduce dust and blade temperature. A garden hose that is set very low is perfect for this. Always wear eye protection when using a circular saw.

Summary:
Set the blade depth of your circular saw to 2 inches (5.1 cm). Attach a 7 inch (18 cm) diamond blade Saw along the lines with the circular saw.