Personally, I would start with something that you want to play. Add cards that work well with it. Then once you understand how those interact with each other, add cards that can make that bond increase. Soon you end up with a list of 40-80 cards. Pick from those the most mana efficient of each variety, and then determine how many you need to reliably find/play them. Build a mana base around it, and then play test it at least 5-10 times without changing parts. Whichever cards ended up staying in your hand, or don't react the way they were inteded get cut, and replaced by something else that can fill the spot.
Building a deck of 1-of in modern is not something I recommend. Nothing will ever fully interact, and the deck will feel a lot slower and clunky.