I've done some seminars in the past and have enjoyed doing them. My first experience was a good one, but man was I nervous. It worked out well though.
When I plan a seminar I plan for these things.
Where will the seminar be held?
Options could include:
In your dojo,
at a friend's or someone else's dojo,
at a VFW hall,
in a park,
at a local YMCA
I don't have a studio of my own so I have done them at other dojos and at the local YMCA's.
What material will the seminar cover?
Depending on where you do the seminar and who you teach it to will change what you teach.
At the YMCA I did a few self defense seminars, so I chose to teach some Kempo material and some Hapkido material. Now decide on what level your participants will be at. I chose very basic physical material in the self defense seminars because most of them had little to no experience.
It was quite different at my friend's Kempo dojo. I decided to do strictly Hapkido and show how it could be incorporated into Kempo. The experience level was higher in this situation than in the self defense seminars so I "raised the bar" on difficulty.
Lesson Plan
Write out exactly what you want to teach. I would recommend writting down more than you expect to teach just in case they are picking things up quickly, or something doesn't work as planned.
One thing that I usually do is give out a review booklet for each participant. So that way they have something to take with them and can look back on it if they forget and want a small reminder.
Price
It's up to you. If you go through a VFW or a YMCA you will have to take care of rental fee for the room from the money you receive from the participants. If you are doing it at another person's dojo I usually run two different methods:
1) Flat fee - Dojo pays one price. no more no less. This puts more responsibility on the host to get people there.
2) $ per head - the host gets a cut and so does the seminar instructor from each participant's entry fee.
If the seminar is far away, you also dicuss travelling expences and/or accomodations with who ever the host is.
I love doing seminars. It's a great way to meet others in the martial art community.
Sorry to be so long winded. I hope this helps.
Take care