Hi....rats are usually tamed pretty quickly unless they are rescues that have been abused,
They just need to feel safe and to know that you are the 'bearer of tasty treats' and they will come to you out of curiosity'
If they are a little skittish begin by talking to them very gently and stroking their back and then offer them a treat as a reward.They have very poor vision so approach them slowly from the front rather than above or behind .
When you first pick them up cradle them close to your chest and cover their head with your hand,this will make them feel safe.If they start grinding their teeth your'e winning as this means contentment in rats ,not annoyance like with hamsters.
Good Luck and enjoy your ratties.They are truly wonderful pets and will give you affection and companionship just like a cat or dog.To bond,its important that they have at least an hours supervised free range time out of their cage a day too.