Draw one large love heart and one small love heart (or however you'd like to do it, maybe several sizes, or maybe only one? I'm not here to tell you exactly how you must do these things.) onto two squares of cardboard, then cut them out. Seal up the bit where you cut into it with sticky tape.
Next, put on your tights and place the stencil on your leg, wherever you think it'll look nice. Paint thickly, filling out the stencil. Make sure the layer of paint is fairly thick, but not goopy. You can then remove the stencil and fix up any edges that look a bit dodgy with the paintbrush. Now do as many of these as you like on the FRONT of your legs. Have a look in the mirror (while trying not to move your legs too much, and avoid bending your kness - walk around like a robot if you have to) to make sure the placement looks good.
When you're done the fronts of your legs, use a blowdryer on each heart for about 20 seconds to speed up the drying process. It's very important that you don't begin the next step until the paint is BONE DRY.
Next, paint over each heart with clear nail polish (in a thin layer). If paint is coming off and going into the nail polish, you have rushed it. WAIT UNTIL IT IS DRY.
Next, use your blowdryer again, or wait for a loooong time if you have the patience. When you can touch each heart and it doesn't feel at all wet or tacky, you are ready to take your tights off.
When taking your tights off, peel very carefully around each painted heart. If there is any paint left on your legs (other than a faint outline), STOP. It is not dry enough, and it won't work. Chill, sit back down, and WAIT another 5 minutes or so (I can't stress this enough).
Once you take the tights off, turn them around and put them on backwards. Begin process again (I told you this would take a while...)
Now you are done. Hooray!