Bend galvanized steel strips, such as metal flashing or heating duct strips, into the desired shapes for your stones. Use one strip for each form, connecting the ends with duct tape once it is bent into shape.
Cover a worktable or plywood board with a plastic bag. Smooth out any wrinkles in the plastic and tape it down so it doesn't shift. Set the steel forms on top of the plastic.
Mix quick-setting concrete with water in a large bucket, following the instructions on the concrete package. For most quick-setting brands you will add water until the concrete is the consistency of lumpy oatmeal.
Pour the concrete into the steel forms. Lay a plastic bag over the top of each form and press on the concrete firmly so any air pockets are forced out. Remove the plastic and smooth the top of the concrete with a trowel.
Leave the concrete to cure overnight or for the length of time recommended on the package. Cut the duct tape and remove the metal forms once the concrete is hard. Let it cure for an additional 24 hours before setting your stones out in the garden.