Before beginning repairs, you or someone you hire will need to clear away any debris and nests in the chimney. To get rid of creosote, the tar-like material that burning wood creates, you will need a chimney brush. You can rent one that corresponds to the flue's inside diameter. Make certain that you seal the fireplace opening before you start repairing the chimney to keep from making a large and near impossible mess to clean.
You'll need to get safe and sturdy footing on your roof, as this is the most common way to clean the chimney. Insert the brush and move it up and down with enough pressure to remove soot without damaging the structure [source: Contractor's Solutions]. If you need a visual for this and have little luck online, you can pop in a DVD of "Mary Poppins," for inspiration.
Anyone's who worked with chimney's is probably familiar with tuckpointing, which is the replacement of fresh mortar for failed mortar joints. Done correctly, with a mortar-raking tool, the process will clean the damaged mortar. If this isn't enough, a masonry chisel and hammer should do the trick [source: Contractor's Solutions, Do It Yourself].
You may want to use a mortar hawk to get mortar into tough places, like the horizontal joints. For vertical joints, you can use a joint filler to spread the initial layer. Sometimes, it's even useful to mix a concrete fortifier or pigment in the mortar so the end result will match the rest of the chimney [source: Contractor's Solutions, Do It Yourself].
You've gathered your tools and materials. Now you can read on to learn more about repairing stone chimney cracks.