Step 2: Places please...
Before you print on the shirt, it is very very important to wash them - I skipped this in the past and it sucked! cotton shrink about 3% after the first wash, so if your shirt hasn't been washed before, it'll shrink a bit in the first laundry and they print will look all thick and would be itchy to wear. Also some cloth factories, put some chemical on the shirts to make them stay wrinkle free and that might make the paint not stick properly (I actually don't know if that's true, it never happened to me and might just be an urbal legend) but do wash and dry your shirts before printing on them.
And now we are ready to begin.
Put a sheet of paper or cardboard inside the t-shirt to prevent the back side of the t shirt from staining.
Spray the back side of the stencil with the spray adhesive, you don't need a lot of it, just a little bit to keep the stencil in place, from my experience - the least the better, even a tiny layer all over the stencil is enough.
Place the stencil where you want to print - since the adhesive is not permanent you can remove it and put it on again as many times as you need to place it, the spray say tacky for long enough to play around.