I agree with the no slimline suggestion. I coudl actually show you exactly why if we could spend about 5 minutes in the shop together. But it comes down to the slimline being the smallest diameter pen. this requires the highest degree of tool control that we come across in making pens. Combining this with a new turner that already has reason to be nervouse and shaky. I know thousands of people have turned the slimline for their first pen, but I still think their are much better choices for the job.
Cigar pens are larger diameter and not that much costly.
I do not agree with the send them to watch videos, unless you want to spend a considerable portion of the lesson time correcting mistaken ideas about what they watched.
Start with covering safety specific tot he lathe. Since it is a woodworking club I would suspect they are already familiar with other shop tools. there are safety issues specific to the lathe. safety glasses of course, proper set up such as making sure nothing will hit the tool rest, making sure tail stock , tool rest, mandrel nut and whatever chuck you are using for the mandrel are all tight and secure. and finally make sure the lathe is on low speed before you start it. some of this is overkill for a pen like the lathe speed thing. But best to start people out with good habits.
My final bit of advice is the tell them three times rule of thumb. It works like this. first tell them what you are about to show them. then show them, then tell them what they where just shown. this little idea was told to many years ago. it is a general rule that people that write, do seminars or otherwise instruct people follow. it has to do with how people are able to absorb information and works very well.
I also would not get to fancy with tool selection. Everyone knows there is more than one way to skin a cat. and just what tool you rough the blank to round with or make the final cuts with etc can become a long list. so keep it simple, you decide what it is best to show them, and save it for another day to discuss alternative tools or techniques. I would also have a pre drilled and glued blank even if I brought an untouched blank to show them drilling. this way you can discuss some of the issues with drilling and maybe even get to demo a blow out on the drill press. but you are then instantly ready to turn a blank without the fuss and mess of gluing. Even if you do not actually drill a blank there. do mention that pecking is required and most certainly point out you are taxing the drill bit beyond it's intended performance. Also warn them about the common mishaps, either blow out or tear out upon exiting of the blank. also have a short list of suggested was to avoid them.
milling the blank. Hmmmm, probably best to demo your favorite but you might also want to have a list of ways to accomplish this as well. no need to show all of them but at least let everyone know that there is not just one way to get it done. None of them are really easy or readily available in the average non pen shop.
Actually mounting and turning. spend some time explaining how to correctly set up the blank on the mandrel. this is a point of error for many turners both experienced and non experienced. not tight enough is a problem, to tight is a problem. otherwise I would not get to far into all the problems they might have.
Don't forget at this point to show you actually follow your own advice on safely setting up and actually check that the tail stock is tight, chuck is tight , blank does not hit tool rest and that lathe is on slow speed. finally point out proper positioning of the tool rest. Ioften see knew people tend to want to set the rest way to far away from the turning. this can actually be dangerous. to far of a reach can cause tools to break.
Whittle out a pen and then you can spend a moment talking about sanding and finishing options. i am not sure I would even finish the pen right then and there. maybe sand it out and hit it with Micro Mesh just to dazzle a couple of folks. in that case I would turn Cocobolo for the Demo though. At least dome type of dense oily wood that will take on a pretty good polish without being sealed or finished.
Actually recently I learned a real good fast finish. I do a CA slurry seal then sand the blank to 600 grit. the barrels are then polished with a buffer. this does not produce quite as fancy of a finish as a full blown CA finish, but it is fast and easy.
Sorry I couldn't think of more, but I am sure others will chime in.
Otherwise, "Break a leg". That is a good thing by the way.