Clean the object you want to make a mold of and correct any imperfections beforehand. A silicone mold will capture even tiny details, including flaws.
Cut melamine board into a square or rectangular piece that you can rest the object on. This piece should extend 1 inch or 2 inches past the object on all sides.
Glue the object onto the center of the melamine with hot glue.
Cut four more melamine pieces, each as wide as one of the sides of the original piece and all 1 inch or 2 inches taller than the object.
Glue the four melamine pieces onto the base piece to form a box around the object.
Apply extra glue to all of the seams of the melamine box. Make sure there are no gaps where silicone could leak out of the box.
Coat the interior of the melamine box and the entire surface of the object with silicone mold release spray. This will keep the silicone from adhering permanently to the box or the object.
Mix enough silicone rubber to completely fill the box according to the manufacturer's instructions.
Pour the silicone rubber slowly into the box at one corner until it is full. Do not pour the silicone directly onto the object, as that could cause air bubbles to get trapped against it.
Allow the silicone to fully cure according to the manufacturer's instructions.
Mark the pieces of the melamine box with a permanent marker so that you could re-assemble it correctly from its component pieces.
Pull the box apart, turn the mold upside down and remove the base melamine piece.
Remove the object from the mold. The silicone will stretch and allow you to pull it out. If the stretching is insufficient, use a craft knife to carefully cut a slit in the mold to create a bigger opening.
Reassemble the melamine box around the mold, with the mold upside down in the box so that the opening is pointed upward. The box will hold the mold in its correct, original shape and hold any slits you had to cut into it closed.