I’ve done more than a few of them. I usually make 2 or more part molds, so I start with a mold frame made of rectangular Lego blocks. You want a shape that you can flip over, so its better to make a fully symmetrical box than to make an odd shape. Once I have the box with at least 20mm worth of margin around the part, I then build a base of modeling clay, press the part in about halfway ( or to a natural separation point) and then use a piece of hot glue gun stick to make a pour spout, and 2 scrap pieces of 3d printer filament to make air vents. I also use a few small marbles to make mold alignment marks.
Once that all looks right, I pour silicone into the mold, covering the piece by at least 20mm. Silicone only sticks to silicone, so you don’t need to prep the part aside from making sure it is smooth.
After the silicone sets (8 hours +/- 4 depending on size, type of silicone, amount of hardener, etc) you pull the completed mold half out, remove all the clay and the marbles, but leave the vent pieces, pour spout form and the part itself. Put the entire mold half back in the Lego frame with the silicone on the bottom
Next liberally coat the entire silicone mold surface with petroleum jelly. This will cause the new silicone mold half you are about to pour not to stick to the one you already made. Pour new silicone and allow it to set.
Now you should have a completed mold. Separate it carefully, trimming any stuck pieces of silicone with scissors. Remove the mold core part, the vents, the pour spout core and then use a sharp pair of scissors to ensure the pour spout and vents are totally clear of obstruction. Your mold is now mostly ready. Usually at this point I cut a set of thin plywood or partical board mold support sidewalls and find some clamps to keep the mold together.
If you did it right, you should have two or more mold sections that lock together using marble shaped tabs. You can now pour almost any castable plastic or resin into this mold
There are a bunch of good tutorials on the process on Youtube or instructables. For a simple part with a flat side, you can just use a plastic bottle a bit bigger than the part, cut it in half, fill it with silicone and press the part into it, flat side up. I generally prefer a two part mold even in that scenario to get a better final surface quality.
Once you get more proficient you can play with adding dyes and metal or stone powders to your resins and make things that feel like metal parts, etc.