This is Hacker News. Some knowledge of hardware is expected.

But I'll describe KiCad's workflow.

First, you draw a schematic diagram. This has blocks and symbols for all the components. You place part symbols, which come from a library. Part symbols have a reference ID such as "R15", which you have to set, and you can also set such information as the actual part number in a catalog, which will be used later. The part symbols library has most standard components, but you'll probably have to draw some symbols of your own. There's a draw program for drawing symbols for new components. (The schematic editor is OK, but the symbol drawing program is a crappy draw program.)

Then you run the design rule checker. This tells you if you forgot to connect a pin, or connected two outputs together. It's sort of like "lint" for schematics. It doesn't know anything about electronics; it's just a topology check.

Then you run a spreadsheet-like program which, for each part, assigns a "footprint" to that part. The "footprint" is the pattern of copper pads and holes on the PC board where that part goes. Many schematic symbols map to one footprint - all 8-pin SOIC-form packages have the same footprint, for example. Sometimes you need to add a new footprint, and there's a draw program for that, different from the schematic draw program. (It's a crappy draw program.)

After that, you click the "NET" icon, and the schematic program generates a netlist. This is a text file with just the part info and wiring topology. You can also get out a bill of materials (BOM) file at this point, for ordering parts.

Now PC board design begins. This is a separate program. You start by importing the netlist. There can be errors at this point - missing footprints and such. You may have to go back to the schematic editor. You can keep both open at once, change the schematic, and generate a new netlist.

At this point, you have a display showing all the parts spread out more or less randomly (there's an auto-place function which does this, not very well), with lines showing where connections are needed. The lines are just straight lines between things that need to be connected, not useful printed circuit board traces. This is called a "rat's nest". Now you drag parts around until you have a workable parts layout. (Some parts need to be close to other parts for electronics reasons. You may want connectors
and such in specific places.)

Then comes connecting up the parts. There's an automatic routing program for this, but it's not very good. (That's what you pay the big bucks for in the high-end EDA systems.) Mostly you draw tracks of copper to connect up parts on the various board layers, and use vias, connecting holes through the board, to connect through layers. This is manual, but the program checks you - you can only connect things that the netlist says should be connected. The program won't let you make a wrong connection. There's also a design rule checker for clearances between wires and such. This is a slow, fussy process. Squeezing in all the tracks can be tough, and previous tracks may have to be moved. The tighter you design your board, the harder this is.

Once everything is connected properly, and the design rule checker says there are no problems, you generate "Gerber files" for each board layer, and a "drill file" which defines where holes go. These are industry-standard files. You send them to a board design service and boards come back in a few days. They're made by photo-etching and CNC drilling, and while it's possible to do this yourself, it's not worth the trouble because custom board fab runs now start around $21.

So that's how you make a basic PC board with KiCAD.