If you find cracks between the bricks on your home, you will need to fill them in. If you don't, you risk letting insects and small animals into your home. In addition, water can get in and rot the wood walls that are behind the brick wall.
Clean the space that needs repairing, using a brush. If you leave little pebbles and dirt in there, the replacement mortar won't adhere well.
Combine some mortar mix with water in a bucket according to the package directions. Alternatively, you can purchase already mixed mortar that comes in a tube that you simply load in a caulk gun.
Moisten the existing mortar with plain water. If you are using mortar mix, work some mortar into the joint with your fingers. If you are using your fingers, you can use rubber gloves to keep your fingers clean. If you are using the mortar in a tube, just squirt the premixed tube mortar in with the caulk gun.
Smooth the new mortar with either your finger or a jointer so that you get a finish that looks like the rest of the joints. Let the new joints dry before allowing them to get wet.
If your joint mortar is a different color than the regular gray, you will need to use the mortar mix. You can get some cement color and tint the mortar to match your existing joints.