For me it's usually as follows:

• Find a card or theme I'm interested in
• Add some other cards that go well with the theme or card I chose first
• Add lands and dual lands for the colors I'm using
• My friend tells me that this card is banned or these ones aren't format-legal
• Start over from step 1

For me, I really enjoy punching up. For Commander specifically, that means playing Red or Mono Red. It's just... no one ever says Red is too powerful in Commander. That might sound silly, but pick a color or pairing of colors you like. It's a remarkably powerful filter that will let you get started.

From there, it's a matter of finding a Commander I like. That usually means a combination of most of the following:

• A game mechanic or win method that my other decks don't already do.
• Low apparent threat level. Usually that means minimal board presence, but it can also mean the real Commander is hiding in the 99.
• "Wow" factor. Does the deck do something that's actually novel?
• Can I let an Opponent 'do the thing', but still stop them from a 'win out of nowhere'? I have a love for what you'd call Aikido moves, things where you redirect an Opponent's move back at them. Not Red's strong point, but they do have a couple (Fork, Return the Favor, Wyll's Reversal).
• Is the deck fun to play against? Not trying to be a pushover, but Commander is a social game I play with friends. I want them to have fun as well.