Cut a piece of plywood about 0.5" larger than the mold in X and Y dimensions. Drill a 0.5" hole in the center of the piece. This will allow you to add weight to the mold to ensure the liquid metal doesn't push the two halves apart. I used a couple of bins of screws (about a pound in total) to weigh down the mold.
If you're prepping to pour your first casting of the day, dust insides of the mold with baby powder and brush off any excess. Then, preheat the mold with a heat gun to around 115° F (double check with the temp gun).
Heat up your metal in a cast iron skillet on a hot plate. I like the hot plates with sealed heating elements so that spills are easy to clean and don't risk pouring down into the plate's innards. When the metal is melted and evenly heated through (check that its temp is above the material's listed melting temp with your heat gun) you're ready to pour.
Turn off the hot plate, put on your heat-proof gloves, position the skillet over the sprue of your mold, and pour in a constant, even stream into your mold.
It takes some practice to get a feel for how much metal you'll need to use to fill the mold. Since the metal takes a few seconds to flow through the gates and fill the mold, the feedback you get from watching the metal level in the sprue isn't a great indicator of how close the mold is to full. That's something you get a feel for after you work with the mold a few times.
Sometimes metal will pour out the sides of the mold or out of the gates, but this isn't usually a problem. Just keep filling the sprue with metal, keeping it as close to the top of the silicone mold as possible, until the leak stops.
Once your mold is filled, set the skillet on a heat-proof surface, and wait at least 5 minutes for the mold to cool. For larger metal castings cooling will take longer, but for something small like this 5 minutes is a good safe cooling time. After that, it's fine to open up the mold to speed up cooling.
Once the metal has cooled off enough to be safe to handle (~100° F), take it out of the mold and allow it to come down to room temp.