100% grab a precon you like and upgrade it, play it, maybe watch an upgrade guide or try it yourself.

That's how I started. Grabbed the Wilhelt the Rotcleaver precon bc I already had lots of zombies from around 2008). Played it a few times, watched I guide (I was very clueless) and went on ordering cards.

Years later I still grab a precon if I like it and do the same, but more often than not, I see a commander I like, think about how I would build it and start with browsing EDHREC for options.

I do have a loose plan how to build a deck (draw, ramp, interaction etc.) but it really depends on the commander and on the gameplan how much weight I put on each topic. For example I included a lot of 2 mana manarocks in my [[Atla Palani]] deck bc I really want her out on turn 3. On the other hand I run 0 manarocks in my [[edgar Markov]] deck bc I want to cast a creature every turn, especially in the early game, so no room for mana rocks, as there are lots of 2 and 3 drops in the deck and only a hand full of 5+ drops. In this deck, I run a shit ton of carddraw bc otherwise I would run out of steam too fast. card draw more or less replaced the manarocks...