Simply patching isn't going to do it. You really need to scrape/dig out all the old, loose mortar, and and repoint all the joints. This will require a number of specialized tools, and knowledge/technique. I'd suggest trying to find a mason who will take it on as a weekend side job. Look for a board of business cards at your local building supply store - especially a masonry supply store.

If you want to take it on yourself, get a good book on masonry. Pictures help a lot in understanding how to do it. Basically, you chip/scrape/wire brush away all the loose mortar, mix up your mortar, wet the brick (for a better bond with the mortar), get a scoop of mortar on a trowel and hold it in front of the joint to be filled, then use a tuck point trowel (looks like an 8" long, 1/2" wide rod on a handle) to push mortar into the joint. The knowledge/techinque is in getting the proper consistency to the mortar (too wet/dry and you'll get a weak mortar), and getting a solid and even fill of mortar into the joints.