To make a stencil you need
a motif
thick foil/cardboard/metal sheet
sharp knife/scalpel
To use a stencil:
stencil
tape
paint
sponge/spray can
a surface to put it on
Find a motif or make your own
Copy/print motif on thick foil
Cut out the black parts carefully
Tape stencil to surface
Apply paint with sponge
Carefully take off stencil
Tadaaaaaaa