To remove the horn ring, disconnect the battery (or the 6 terminal wire plug that goes into the column) and press down and turn counterclockwise about an eighth of a turn. Depending on the strength of your spring, the horn ring can be a challenge to get on and off.
If you are seeing sparks come out from under your horn ring, it's a good bet that your contacts are worn out or you possibly need to install a relay. Most horns draw a lot of current. A horn ring contact kit is available from just about every Mustang parts house out there for a few bucks. Seems like a logical place to start.
Good luck, horns are evil by nature.
