Start a New Project: In Flux, begin by starting a new project. You can do this by clicking on the "New blank project" option in the menu or by using the ALT+N keyboard shortcut on Windows or ⌥+N on MacOS[1].

Add Parts: Use the Flux library to add the necessary components (transistor switch (MOSFET), diode, inductor, and output capacitor) to your project. If the library does not have the component you are looking for, you can create a new part or import one from another tool[2]. Each part has a schematic and potentially a symbol, simulator model, or PCB layout (footprint)[3].

Create Circuit Schematic: After adding the components, the next step is to create a schematic of your buck converter. This involves connecting the different components according to the buck converter design. For instance, the MOSFET and diode should be connected on the same node with the diode anode connected to GND. The other side of the MOSFET should connect to the input voltage through an inductor, while the cathode side of the diode connects to the output voltage through a capacitor.

Create PCB Layout: Once your circuit schematic is ready, you can proceed to create the PCB layout. This involves defining the footprint for each component and placing them correctly on your PCB. You would also need to route the paths for your circuit. In order to accomplish this, make use of the powerful layout rules and design capabilities provided by Flux[3].

Simulation: If desired, you can also simulate your design using the built-in simulator to verify that it behaves as expected[4]. This is especially important for a buck converter where the input to output voltage ratio, duty cycle of the switch and component values need to be verified to ensure they meet the required specifications.

Publish to the Library: When your design is final, publish it to the Flux library, so that it is accessible for future use and can be shared with others if desired[5].