What I did was traded/signed all the players I planned on having on my team to the Lions (the team I replaced) because if you don't do that then all the players on the lions will disappear from the game.
So when you do all trading and signings (make sure you write down all your players to make the next step a little easier) then you go and create your team. When selecting the team type select fantasy. You will then go into a screen where you pick your players from the entire player pool. This is the hardest part because they aren't grouped into teams so what I did was went through each position and searched by name.
Keep in mind you will select 50 players so before you do that make sure you have 50 players on the team you will replace.
So just a little example I have C. Williams as my running back so what I did was traded one of the Lions running back to the Bucs for C. William.
Now I didn't think trading K. Smith would be fair so I released K. Smith and picked up a running back close to Williams.
Same with my other big named guys (Calvin Johnson, Ernie Sims). They are currently sitting in the free agency list.
Hope that helps a little