To get to overall 99 with a created player, the route I chose was the running back route. Create a Power Running Back for any team, just make sure you make your running back an instant starter. I kept the rest of the settings, including the difficulty, which is pro. This will unlock "Overall 80", "Overall 85", "Overall 90". I then simmed to the first game of the season and changed the sliders to everything in my favor. Make sure to put all CPU sliders on 0.
Now, play every single game of the season, but sim the pre-season and post season. (Make sure to win all post season games) By the middle of the third season, assuming you have played every game, you should have beaten both the all time rushing yards record and the all time rushing TD record. This should net you a ton of XP and Legacy score, as you will being winning the MVP and the Super Bowl along with other rewards.
I recommend using the Chiefs playbook, and then going into: By Play Type > Run > Counter > And then choose any counter play.
By this time, I had well over 200,000 XP, probably closer to 300,000 XP.
Now, raise these stats to these levels:
Speed - 95
Acceleration - 91
Awareness - 98
Ball Carrier Vision - 97
Stamina - 79
Thanks Daft Assassin
Nappster187 also said that if you upgrade strength, this will help your overall. (Physical Grade)
By this time, you should be at least a 96 to a 98 overall. Your production might not be in the high 90's quite yet, so you might have to play some more games to raise that. The stats that you need to raise into the high 90's are Physical, Intangible, Durability, Size and Production. To pass Emmit Smith, you need over 13,000 Legacy Score.
This should take about 3 seasons, and should take no longer than 4 seasons.