Cleanup

You should start by cleaning up your prints. Whiskers hanging off your parts or overflowing blobs of material should be trimmed off with an Xacto or matte knife.

Mold Assembly

Assemble the crystal mold according to this diagram. The top portion should simply snap onto the bottom with just a little bit of force. Getting it back off after everything's cast should be easily done by hand. If you're having trouble, slip a flathead screwdriver underneath one of the arms of the top half to pry it up.

Collect Casting Materials

Gather everything you'll need to cast the silicone in one place. This means finding your silicone, gloves, stirring sticks, measuring cup, scale, and mold.

Measure and Mix

Measure and mix your silicone. Any two-part RTV silicone will work here, but I'm using a 50/50 ratio rubber called EcoFlex 00-50 because I happen to have it on hand. The mold only requires about 15g of material to fill, but you should mix up 20 to 30 grams of material just to make sure you have a little to spare in case you spill or a lot sticks to the side of your cup.

I use a vacuum chamber to degas my silicone to make it extra clear and bubble-free. This isn't an essential step, but it adds an extra bit of polish that helps the ring look professional.

If you've never cast anything in silicone before, this is an excellent tutorial on mixing, degassing, and pouring rubber.

Fill Mold

Pour your silicone through the holes in the top of the mold. When it's filled to the top rotate the mold around and tap it gently against your work table to make sure you work out any trapped bubbles. If you accidentally overfill the mold and silicone is flowing down the sides just wipe it down until the silicone's flush with the top.

Don't worry about a little bit of over or under filling. You can always trim bits away if they're preventing your ring from snapping together during final assembly.

Cure and Demold

Let your silicone cure overnight. I keep the cup everything was mixed in around to poke at the bottom and tell when everything's cured so I can cheat and pop parts out ASAP. Some brands of silicone will cure much faster, but everything's guaranteed to be cured and safe to demold in 24 hours.

All you have to do to get the silicone out of the mold is pop off the top and gently pull the silicone by a corner. It should gradually come away from the mold. Now's your chance to clean up any dangling bits or thin sheets of material that crept between the two mold halves (this skin is called mold flash).

If your silicone is tacky and you're worried about it picking up dust from the environment you can dust the part with talc, but if your part is fully cured any dust that settles on it should be easily removed with a damp paper towel.