What I did was I cut out a paper heart the size I wanted the clear heart shaped 'window' and taped the edges of it. I then taped the heart onto the jar. Now you paint.
I just used some old acrylic paint I found around. First you paint the whole jar white. Except for the heart, obviously, you paint around that. Once that drys (or is mostly dry for you impatient folks like me) you get it paint it pink, or red, or purple, or whatever color or colors you want!
Once you've finished painting you get to make the goodies for the inside!
For the tall purple jar I filled it with mini no bake cookies. It's for a younger girl and she literally LOVES no bake cookies. For something special I topped them with, ahem, organic sprinkles. Not a word.