it usually is easiest to do this at the end of a season... Thats when players get released...
You need to search by team and go to unattached players in the bottom right hand corner of all the teams flags...
I have picked up such players like Rooney and C.Ronaldo floating on there after being released.... When you select a player off this list it should prompt you with 'negotiate contract' which means you just have to offer them salary points!! Easy - good way of making money too, get them and sell them or use them as collateral for other players in trades! As you progress in Master League aswell (im talking years on like 2009+) this is where you pick up all your promising youth players!!