Take one sheet of shrink plastic (I use Lucky Squirrel shrink plastic) and using a template (I used a small roll of sellotape), draw yourself a circle on the shrink plastic with a permanent marker (fine) about 5-6cm in diameter. Cut out your circle.
Using a hole-punch punch two holes in your circle, as above.
Using a fine permanent marker draw your design onto your buttons.
Put your shrink plastic buttons onto a piece of thick flat cardboard and put into the middle shelf of a pre-heated oven (180 degrees). Your buttons will start to curl and shrink - this is quite a fun process to watch, they only take a couple of minutes to 'cook'. By then, they will have shrunk to about half their original size and they will be much thicker. The buttons will flatten back out themselves after their curling an shrinking, then you can remove them from the oven and let them cool.
(To make your buttons waterproof or suitable for sewing onto clothing, you will need to add a glaze to them. I use Diamond Glaze which dries clear and shiny. I paint a small amount onto the surface of the button and leave to dry for 24 hours. I find it is best to leave your buttons for a few days after cooking before applying the glaze to make sure the marker is fixed so it doesn't smudge.