To Make The Fondant Glue:Place one mini marshmallow in a small dish with a few tablespoons of hot water. The marshmallow won't melt completely, but the sugar from the marshmallow will start to dissolve in the water and act as a great glue for sticking things to the fondant decorations.

If you don't have any marshmallows, you can dilute a little bit of corn syrup with water.

Be cautious when using fondant "glue". You want to use water very sparingly around fondant, too much moisture may cause drips or meltage.

Assembling The CakeAfter you make all your fondant decorations, I recommend letting them sit out to dry for a few hours or overnight. This will make them much easier to move and manipulate on the cake.

Before placing the decorations on the cake, I suggest sketching out your design or practicing the placement (for the top decorations) on a clean cake pan. Just to get the idea of what and where things will fit best.

When you're ready to assemble, make sure the cake is completely frosted and that the buttercream is still soft.

Start with decorating the sides of the cake. Add the jellyfish first since they're longer than the other decorations. Secure the body first to the side of the cake and then arrange the legs however you like.

To stick the decorations into the frosting, you want to gently press the fondant pieces into the soft buttercream, but don't sink them too far so that they get buried in icing. Gently press them enough that they stick firmly to the side of the cake and don't fall off.

For the top decorations, the turtle and octopus will sit flat on their own. The coral you'll have to sink in the frosting a little to keep them standing. You can also pipe a little buttercream mound on the top of the cake for them to sit in.

For placing the fish, the big fish will sit on the cake easily and I recommend sticking smaller fish on a toothpick to give them some height and to look like they're swimming around.

I added a starfish family (per the birthday girl's request) to the top of the cake in addition to the ones on the side. I piped little mounds of buttercream for them to stand in and I piped a little frosting behind the starfish for back support--to keep them from tipping over.

Once decorations are placed, allow the cake to chill and set in the fridge until firm.

3-4 hours before you're ready to serve, allow the cake to come to room temperature.

Slice, serve, and enjoy!