For this achievement you must sign a player whose average is at least one higher than your current best player. This achievement is very difficult to achieve when playing with Barcelona for example, as they already have some of the best players in the world. So pick a lower rated team when going for this.
If you really don't know how to get this, start a Career at FC Utrecht (Dutch Eredivisie) and buy Willie Overtoom (midfielder) from Heracles (Dutch Eredivisie as well). I got the achievement this way as well.