Measure out 25g of Plaster of Paris for each silicone mould you are going to fill, then add 10% for wastage.
Prepare the Plaster of Paris as per the packet instructions in a disposable container and mix with a disposable spoon.
Lay out your silicone moulds on a tray or bread board (if using).
Fill your silicone moulds to the halfway point (it doesn't have to be perfect, just eyeball it).
Set your plaster-filled moulds aside to set.
When the plaster is dry, peel back the silicone moulds to release your plaster flowers.
Paint and decorate your plaster flowers as desired
Glue a craft magnet to the back of each flower.
Give the glue at least 24 hours to harden and then you can stick your floral plaster magnets to your fridge.
Notes
Prepare your wet plaster mixture as per the directions that come with the plaster of Paris. If there are no directions, then a general rule of thumb is to mix plaster and water at a ratio of 2 parts powder to 1 part water.