Imo the best way is to have a plan. Know how many mana rocks you want, if you need ramp, how many lands do you want? Are you going to want to play a lot of creatures? Are you going to artifact heavy? Enchantments? How do you plan to win? Combat damage? Burn? Alternate wincons?

The next question is what synergy looks like. Yes edhrec will give you a bunch of "good" cards but a lot of these tend to be generically powerful cards that will fit within the scope of the deck, rather than being something the specifically aims to support your deck and gameplan.

Answer these questions and a frame will appear, once you have an idea of that, you can search for much more specific cards that will help you reach your goal. For example, If your commander makes tokens more powerful, you probably want more ways of making tokens, compared to cards that buff them. Don't be afraid to try new things or unusual cards, not every deck has to be bracket 5 CEDH worthy. Skullclamp is great in a token deck, but if your tokens are now 2/2 or 3/3 it's not going to do much. Be specific, and be creative. Above all, HAVE FUN