To begin using a foam brush or a small paint brush paint the exterior and interior with 2-3 coats of acrylic paint in your choice of color. Allow the boxes to dry between coats of paint.
When the paint has dried spell out your own conversation heart message (such as love you, kiss me, be mine, xoxo, hot stuff, only you) on your box using red letter stickers.
Here are my two boxes all done. While I went for more classic conversation heart messages, you could easily customize it with messages perfect for friends too.