Angle grinders come in a variety of sizes -- from 4-inch blades up to a whopping 11-inch blade -- and are typically used for grinding down or through various materials that are unsuited for cutting with regular saws. If you need to make small cuts in concrete, an angle grinder is perfectly suited for the job -- although larger, deeper cuts would be easier with a dedicated concrete saw. Be aware that there will be lots of dust generated when you make the cuts.
Install the stone disk onto your angle grinder. Remove the existing blade first if there is one already installed. Stone disks are designed for cutting through stone and stone-like materials, and one will be needed for this job.
Put on your goggles, heavy gloves and dust mask. The angle grinder will throw up a lot of dust in the air and may also throw up chips and shards of concrete as well.
Position your grinder above the area you wish to cut, then switch on the grinder. Make sure that the disk is not touching anything when you power up the tool.
Lower your grinder's disk onto the concrete and apply a medium amount of pressure to start grinding through. Make sure you hold the grinder with a firm grip, as there is a tendency for the grinder to want to jump out of your hands when it hits the concrete.