Generally for running clinics, there's a few things you need to know. First of all will your barn let people trailer in? Are there overnight stalls available? Is the arena and facilities suitable for the kind of clinic you want to run? How mush will it cost to "book" the facility for two days? How much is the clinician's daily rate? Travel charges? Accommodations? How many people will she teach in a day?
Once you have figured out all the costs (ie. $1000 daily rate, $500 travel, $200 accommodations, free facility, will teach 8-10 riders), then you figure out how much each lesson will cost (ie $2700/8 = around $325 for two lessons) how much you want to charge spectators etc..
I always find that it is a great idea to take deposits of around $100 per lesson just so you don't have people backing out last minute and then you're stuck paying the cost out of your own pocket. And for spectators, any money you get from them is gravy. If you decide to run it NFP (not for profit) then divvy up the money you make from spectators to the riders. Or you can keep it as profit.
Then as far as for making posters you'd want to put on it who the clinician is and a short title (FEI "I" judge, Level 3 EC dressage coach, etc..). Put on how many lessons there are (ie - 8-10 a day), what they cost and if people are expected to take 2 or not (phrase this like $150 for one lesson if you don't mind people taking individual lessons, but if you want everyone riding both days then say $300 for two lessons on the poster). Also indicate if there are outside riders coming in, what the haul in fee is and if there are stalls available. Also indicate that a deposit will be required if you're going to do that and that spectators are allowed/not allwed and free or $25 or something. Don't forget to leave contact info on the poster.