It takes time, and the self control not to kill all those minions ;)
You'll find most answers in this Lol Forum topic
Long story short, you shall kill only the ennemy's mage minions first. They are the weakest, but they hit the hardest (twice as much as the melee minions if I remember correctly). Your minions will spend quite a bit of time to kill the solid melee minions, just enough time to let the next wave reach them, you'll then have 4-5 minions left in your first wave + the 6 (or 7) of the next wave. If the initial fight is in your part of the map, you may eventually kill the ranged minions of the opposite second wave, in order to build a hardcore minion wave, but it takes time.