Phone up the secretary of the hunt you would like to go with, tell them you would like to go as a guest, they'll tell you when and where the meet is, how much the cap is, and you can also ask them what to wear, how to turn your horse out etc. If you're going as the guest of a member, get them to phone up.
On the day dress your horse and yourself up, drive/hack to the meet in plenty of time, introduce yourself to the master, find the person collecting the cap and pay up. Then have a glass of port, a sausage roll and have a nice day!