First, I'd like to cover some mold making basics to help you generate working molds right off the bat. I use SolidWorks to build CAD, but this method should be easy to replicate in Fusion360, OpenSCAD, or any other program designed for making mechanical components.
The simplest mold I know of is the 1-part open mold. This type of mold mold (example below) is an open cavity with the negative impression of the part you'd like at the end embedded in it. When you pour a liquid material into a mold, it wants to find its level just like water would, so your part is going to have a flat surface wherever the liquid finds that level. If you have cavities inside your mold that trap air, your casting material will be prevented from filling that area just like the s-bend in a toilet prevents the water in the bowl from running down into the sewer until it is flushed.
The large top-hat-shaped mass in the middle of the metal casting pictured below is a well, and the branches leading from it to the stair-stepped ring are called runners. When dealing with a material that you heat up to a liquid state to pour, and then wait for it to cool and freeze to unmold (like wax, metal, chocolate, or Jello), it's important to manage exactly how it flows.
If your casting material cools before it's had a chance to push all the air out of your mold, it will freeze in place and prevent any additional hot material you're pouring in from pushing past it. This is why having a well in your mold (a place where there's a big thermal mass where everything flows out from) helps materials like metal form a good casting. Since there's such a sharp transition between liquid and solid metal, it's helpful to have it as hot as possible for as long as possible while it fills your mold.
However, you wouldn't want to connect your well directly to your part, since you'd have to cut or grind the well away from it at the end of casting. This is the purpose of runners. Runners take hot material from the well and distribute it evenly to the part. Runners are designed to be thick enough to prevent material from freezing or restricting as it passes through, but thin enough to be easily separated from the part when it's being finished.
After trying some experiments with a 1-part mold, I decided to optimize the design a bit more. The challenge was that the metal on the top surface of the mold wasn't ending up flat and even like I'd hoped. I could definitely make these parts work for my design by flattening the back with some sandpaper and snipping off the excess material with clippers, but I thought it would be even better to design a 2-part mold to save anyone replicating the project the trouble of doing all that hand labor themselves.
Below, you'll see a picture of the final design. This mold has a few additional elements besides the well and runners. I extended the well upwards through the new second mold half, forming a hole in which the metal gets poured. This entrance hole is called a sprue. I also added small vent holes at the four corners of the part to allow air to escape as the mold is being filled (remember that the metal being poured in will need to displace the air inside the cavity now that this 2-part mold is totally enclosed). These small vents let air out, but when the metal hits them, it cools quickly. This makes for a neat little mechanical action where the metal pushes out all the air, but can't escape, itself.
This updated mold also has keys (the troughs going around the metal part in the picture below, which are mirrored in the silicone part on the left). Mold keys make sure the mold components are aligned during casting. Since this design is rotationally symmetrical, it's alright that these keys are also symmetrical, but if your molds need to lock in in a very specific orientation, make sure you design your keys to only allow it to fit together one way (I've been burned by that one before).
I also designed labels right into the mold - engraving a mirror image of the text I wanted to appear on the silicone molds into the 3d printed parts. This helps keep version numbers straight. Given that printing your molds allows for iterative prototyping, it's easy to generate a bunch of molds that look a lot like one another and it's helpful to add in features that help prevent mixing up mold components or casting a deprecated design.