spend lots of time by their cage and talk to them so that they get used to your voice. you'll know that they are used to your voice when they are comfy enough to run around their cage when you are near them. leave the door open and offer them treats now and then, i hear meal worms ( preferably live, but if you are squeamish you can get them dried out) are irresistible for mice / rats. start to handle them one by one as often and as long as you can. I normally use my jumper pockets for them to hide in while i sit and watch tv or walk around the house.
just devote as much time and attention as you can to them and every time you are able to handle them, treat them for good behaviour.
what ever you do, do not force them to be held as that will scare them away from you.
soon enough they will be holdable, time it takes depends on how old they are.
And on the effort that is put in.