Start by building a nice bit of texture for the background. As we are not on the street, we literally have a blank canvas, so add whatever you want. Wheat paste your chosen texture onto a canvas, or wood or whatever you want. You don't need to buy any of the materials - if you wanted, you could paste paper napkins onto a bit of found wood. I chose record dust sleeves & one or two 7" record sleeves too, as I have 1000's knocking around in the studio as a result of stencilling onto vinyl records & pasted them on to the canvas.
The paste will need to be strong, try adding a table spoon or two of sugar to add strength. So paste your texture on, and if you want to wrap it around the canvas, you can staple your texture to the reverse, as if it were part of the canvas. Think about which bits are going to be seen, and try
----------Then let it dry----------
Then put your masking stencil down, if you need to fix it down, do so with some low-tack masking tape, (as fixative could rip off too much of the texture) and roll on acrylic paint onto the background, to leave a nice negative space.
----------Then let it dry & remove it ----------

Now the final bit, carefully place your stencil over the negative space, and give it a good spray / roll of paint. If the stencil is very fragile like mine was, gently roll on acrylic paint.
----------Then let it dry & remove it ----------
So that's my technique. On the streets, it is a lot easier, as the texture already exists - its on the walls, the fences, the little green boxes.. But you have to complete the whole process in a couple of minutes, so you could say that it's not as easy at all....
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