Remove any loose bricks that are not completely free from the porch using a cold chisel and hammer. Chip away the mortar joint and pull the old bricks out. Use the chisel and hammer to even out the empty space as well.
Combine 1 part cement with 3 parts of masonry sand in a bucket, then wet the mixture using a hose. Stir the mixture thoroughly with a trowel until it looks like mud and has the same consistency.
Squeeze concrete bonding agent out of a tube or pour it out of a can into the empty space where the brick was previously.
Scoop some mortar out of the bucket and spread it on the bottom of the empty space, then set the old brick back into place, if it is not cracked or chipped. If the old brick is damaged, set a new brick into the space.
Fill the gaps on all sides of the brick with mortar and let dry for at least 24 hours before walking on that space. Repeat this procedure as needed for other loose bricks on the porch.