A cup is a complicated shape to mold because of the large opening at the top, but even if you have no prior molding experience, you can tackle this difficult shape if you take your time. The cup requires a three-part mold instead of the two-part or one-part molds usually used with simpler shapes. Two parts of the mold shape around the outside of the cup while the third shape fills in the inside of the cup.
Purchase a water-sealed mold container with a detachable bottom from your local art supply store. The mold container holds the mold rubber until it dries, forcing it into a simple square shape.
Fill the bottom of the mold container with clay. After you apply an inch of clay on the bottom, set a cup (on its side with the handle resting horizontal to the bottom of the mold container) on top of the clay. Rest the cup so its opening rests right against the wall of the mold container.
Fill the container with more clay until it reaches halfway up the cup. The mold should also reach halfway up the handle of the cup. Smooth the surface of the clay with your fingers so it has no cracks or hollow bubbles that will allow the molding rubber to seep around the cup.
Poke small 1/2-inch deep holes around the sides of the clay (at least 1/3 of an inch in diameter). These indentations will form small teeth in the mold rubber, which will help connect the two sides of the mold together.
Spray the cup and the clay with mold release. This keeps the clay from sticking to the mold rubber when you separate the two.
Pour the mold rubber into mold container until it completely submerges the cup. Leave about an inch of space between the surface of the mold rubber and the cup. Let the mold rubber dry for 24 hours.
Flip the mold container over and remove the bottom. Scrape out the clay without disturbing the cup.
Spray the cup and the dried mold rubber with mold release. Fill the vacant area of the mold container with liquid mold rubber. Leave it to sit for 24 hours.
Slide the mold container off the dried mold rubber. Turn the mold right side up so you can see the open part of the cup. Scrape out any mold rubber that leaked into the opening of the cup.
Construct a new mold container from acrylic plates. The plates must meet up with the sides of the mold as the mold stands right side up. The plates must extend upward past the top of the dried mold rubber by at least 1 inch.
Glue the acrylic plates together using a hot glue gun. Use enough glue to make the corners of the plates water sealed. Spray the opening with mold release.
Fill the top part of the mold to the brim of the acrylic plates with mold rubber. The mold rubber should seep into the opening of the cup and across the top of the mold. Leave it to dry for 24 hours.
Pull apart the acrylic plates to gain access to the mold. Lift the top section of mold away from the two lower sections. If some mold rubber seeped down the outsides of the bottom molds, gently peel it away with a craft knife.
Pull apart the two bottom sections of mold carefully and remove the cup. Set the cup aside and reassemble the mold. Drill a thin hole through the top section of the mold until you reach the open cavity within (this is where you will insert the casting material to make a new cup).
If you can't find a mold in a nearby craft store, you can make one. Just glue four pieces of clear acrylic together to make a square large enough to accommodate your cup. Attach a piece of acrylic to the bottom of the square with glue. Scrape the glue to remove the bottom when the time comes.