The stencil design trend seems to be becoming more mainstream and a little trendy. Photoshop can make real easy work of your stencils.
First, let's start with the image we want to make a stencil of. You're going to want to go to your Channels palette Window > Channels and try to find the channel that has the most contrast. For me, it was the Blue channel.
Right click on the channel and choose "Duplicate Channel..." You can name it whatever you want. With your duplicated channel selected, bring up the Levels dialog Ctrl + L. Play with the sliders until you get a decent edge and enough of the detail you want.
Hold the Ctrl key and click on your new channel, this will bring it up as a selection. Press Ctrl + Shift + I to inverse your selection. Now click on the RGB (or CMYK) channel.
Now go up to Edit > Define Brush Preset.... Name it whatever you want, just as long as it's something you can remember.
Now you have your brush stencil made. Go to whatever document you want and select the brush tool B. Go up to your Brush Presets in your Option Bar and scroll to the bottom... you should see your newly created brush.
Now you can just paint your 'stencil' on like a normal brush. You can even adjust the size by using the { and } keys.
Repeat the process as much as you want and start doing some creative collaging.
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