You'll need your image to be plain black and white and also make sure it's the right size for your shirt. Multi-layered stencils are possible but we won't be covering them in this guide.
The plastic isn't strictly necessary but it's unlikely that paper will withstand the high pressure spray paint. It's also going to allow you to cut more detailed stencils without it breaking.
You could wear your shirt right away but this next step will help to set the design so you can wear it again and again without it fading.
There you go! You can now wear your shirt wherever you go, happy in the knowledge that there's none other like it. I hope this guide was interesting and maybe now you can go and make your own shirt!