Bikini in main house color (in this case, navy blue)
Printer or Sharpie and tape
Sparkle paint in secondary house (in this case, bronze, as per book canon)
Stenciling brush or sponge
Gryffindor Crest, by Jennifer Ofenstein (a.k.a. ofenjen)
Hufflepuff Crest, by Jennifer Ofenstein (a.k.a. ofenjen)
Ravenclaw Crest, by Jennifer Ofenstein (a.k.a. ofenjen)
Slytherin Crest, by Jennifer Ofenstein (a.k.a. ofenjen)
Measure the space on your bikini top and bottom to know what size your images can be.
Open the images on your computer and adjust their size until they are big/small enough to fit on the bikini.
Either print a mirror image of the stencil on the paper side of the Mactac or tape the Mactac to your computer screen and carefully trace your stencil on the sticky/transparent side.
Using an exacto knife, cut out your stencils, making sure to leave bridges if need be. If you don't like working with bridges, make sure to keep all the island pieces! Please note that you should only cut the portion of the Jennifer's stencil where you can read the house name (i.e. don't start cutting the badger, the lion, the eagle or the snake as well as the other details, unless you want those on your bikini as well).
To avoid the paint cracking when you wear the bikini after, you'll need to stretch the fabric evening while you're stenciling. This can be done by using a mannequin, or by having someone wear it. Due to the stencil placement, make sure that person is comfortable with what you'll be doing. Please note that if they wear something under the bikini, it will most likely get stained with fabric paint (especially the bottom piece). You can also place the piece on something that is about the same size as your waist/bust, but the results won't be as good.
Once the bikini is being worn/stretched, stick the stencil to the fabric and apply the paint using a stenciling brush or sponge. The top stencil may not stick perfectly due to the round shape, so you will have to re-stick it as you add the paint.
Dry the stencil a bit with an embossing iron or hair blower while it is still being worn. Be careful not to stick it too close to the Mactac, to avoid melting it. Carefully remove the Mactac and if you're stenciling on someone, have him/her very carefully remove the bikini top/bottom.
Let dry completely and iron as per fabric pain instructions to heat-set the color.
Josée Leblanc's tutorial was taken with permission from Harry Potter Crafts.