Choose the image you want to stencil. I did a mixture of negative stencilling (when you spray over a block image) to do the ponies and positive stencilling (where you spray through holes to create an image) for the text.
I cut my stencils from acetate using a hot knife, but they can be made from regular card using a craft knife or scissors too.
Lay out your stencils on your vest/ t-shirt and spray over them (this isn't me, this is one of the other participants at the workshop, but she's used the same ideas as me - negative image, positive text.)
Let the paint dry, the put a sheet of paper over the design and iron to fix the images. Wash by hand for the first few washes.