If you don’t have a stencil (available from craft shops), why not make your own? Overhead transparency sheets are very useful for this as they are sturdy and reusable. You can of course just use cardboard. Print out the letters or numbers from a computer program and cut them out with a craft knife, leaving the area around them intact.
Place the stencil flat on the tin and secure it around the edges with painter’s tape. Use a stencil paint brush/sponge brush to dab the white acrylic paint onto the tin, working from the centre of each character out towards the edges. Make sure you catch any drips that will make your work untidy.
Leave the painted characters to dry before removing the stencil. This will prevent any smearing of their edges.