all you need is some card, a craft knife and some spray paint. i used car spray paint because it was the only kind i could find but any will do.
find a picture you like and begin turing it into a stencil. i used a make-up advert and i believe its kate moss' face. i used tracing paper to draw out the eyes nose and lips in an abstract style. making sure to leave gaps inbetween for the eyes and lips otherwise when you come to cut it out you wont get the lines you want and there will just be big random holes. this can take some working out with more detailed designs. this is why its done on tracing paper first.
when happy with the design and that it will work the way you want it to then trace this onto some thick card. using the craft knife cut way the bits that you want to be revealed. be careful doing this and you must have patience because it may take a while to get the sharp clean lines and this cant be rushed.
once you have finished cutting out your design your ready to spray. get the t-shirt or item of clothing you want to spray and put card behind the area you want to spray to make it flat. then place the stencil on top. then follow the instructions on the can and dont forget to shake it before spraying. always remember to spray quite a distance away to get and even effect and not to get the stencil too wet by spraying up close.
you may want to try this on some paper before doing it on your clothes and you can make any modifications to the stencil if your not pleased with the way it looks.
and hey presto one grafitti stencil top/garment
hope that helps if its not so clear feel free to pm me any questions