Watching games first and foremost. I also look at DraftBuzz, PFF and ProFootballNetwork. But often they're way off or stating something that is entirely false about a player. Just an example, they all said Tyler Warren couldn't block in the run game and was a 2nd or 3rd round pick in December which blew my mind on how inaccurate and wild it was. Now they all have him going top 15, and some have him going top 10 and claim he's good at run blocking. All along I've been saying he's the best pass catcher in this draft that can also help in the run block game.
Another is in December they were all calling for Dart to be a possible day 3 pick and I thought that was garbage. I even posted a couple times about it. Then the Senior Bowl comes along and now's he's a 1st round pick and some even have him going as high as 3rd overall to the Giants yet those websites still state he's a day 2 pick. I would be more surprised if he doesn't leap Sanders to be the 2nd QB off the board, and has a possibility to be 1st overall if he puts up a strong combine/pro day.
It's almost as if those websites guess a lot along the way, and get updated by scouts as the process goes on and never watch the players themselves.