They WILL find their own areas to dust bathe. Sometimes it's nice to have a dust bath in a sheltered area for the birds for when it gets too wet in their favorite spots, though.
I'm sneaky and find their favorite dusting holes in order to add a few handfuls of food grade DE as well as some wood ashes to them every so often.
Here's Harvey Ussery's article on
making a dust bath for the poultry house. I love his articles. This one was in Backyard Poultry a while ago... I forget what month.
I'm sure you could get very detailed in your planning; what I can recommend from my personal experience is that
1. Your bath should probably be big enough for several birds at once, if you have space, because it seems like they like to dust bathe in groups, for some reason.
2. It should definitely have a lip to reduce the amount they toss out. I suppose it could also be very high, but giving it a lip also provides Gloria something to perch on while she decides how she's going to drive out MayBelle, who's been taking far too long a turn.
3. I've also noticed my girls like to dust bathe in patches of sun, even when it's quite hot out. I suppose that will only get more important in the winter, so if the bath could be placed in front of a window, it would be the height of luxury, I'm sure.
Then again, it's just a box of dirt. When the girls were little, they had a fanTABulous time breaking up a couple lumps of clay I brought in for them.