Find a hall and get a model. Simple as that. 😉 the basic costs are hall and model fees, say fifty quid to cover that and maybe a fiver’s worth of tea and biscuits. Between five artists, that’s expensive, but get seven and it starts to get fairly easy. That’s based on a three hour class, for two hours the costs come down to thirty five to forty quid.
There can, and probably should, be additional expenses. A fan heater is a must in Northern climes, for example. Then any props and spotlights that are needed must be budgeted. Natural lighting is usually fine, though, and most models can do a lot with a chair and their dressing gown belt.
There are other factors to consider besides the finance. A lot of the models have day jobs and/or kids, so can’t make some times. Putting a class on on a Saturday means it’s more difficult for working mums to get there (or working dads) either to draw or model. But a daytime class during the week restricts others, so it’s a compromise of course. If most of the artists are going to be retired people, a daytime class is best. Younger artists who work during the day might prefer an evening class, but the timing is never going to suit everyone.
Beyond that, a sign on the door warning visitors not to enter and making sure nobody is liable to peer in through the windows are all that’s needed.