As solid as they are, brick walls are not impervious to the elements and they eventually show signs of wear and tear. A frequent problem with brick walls, especially those from before the 1930s, is crumbling mortar. The process of fixing this mortar is called pointing (sometimes "repointing"), and it involves basic masonry skills.
Check out the age of the wall to be repaired. Prior to the 1930s, brickwork in America was done with lime and sand mortar mixes and softer bricks. If you point a soft brick wall with modern Portland cement-based mortar (which is very hard), you will cause cracking and spalling in the brickwork and create a much bigger problem. If you are in doubt, point it with a soft lime and sand mortar mix.
Rake the crumbling mortar with a hand sledgehammer and cold chisel. Anything mortar that you can disturb with your fingers should be removed. Work in small areas so as to not undermine the wall by removing large swathes of mortar at once.
Mix your mortar in a bucket or wheelbarrow, depending on how much wall your are pointing. Follow the recipe by adding as much water and sand as directed.
Scoop up mortar with a trowel and pour it into a mortar bag, or "cake decorator."
Place the cake decorator's nozzle into the joint and fill it with mortar by holding the bag up and squeezing so gravity and pressure force the mortar out.
Press and compact the mortar deep into the joint with a trowel. It may be necessary to apply more mortar to the joint afterwards to finish filling it. Then shape the mortar with the trowel so it matches the old mortar in the surrounding wall.
Allow the mortar to harden for roughly half an hour before returning to scrape away excess mortar with a stiff brush and the trowel.
Repeat Steps 4 through 7 on small sections of wall until all of the soft and crumbling parts of mortar are pointed.