Take a spray of millet or a handful of cockatiel seed in your hand and just leave the hand in the mouse cage while you concentrate on something else.
The reason for using these particular treats is that mice love the seeds, but since they are small they have to keep coming back for more. Do this as often and for as long as you can stand – it may take several attempts before the mice start to take food from you, but eventually they will start to climb over your hand and perhaps try to climb up your arms.
Once your mice start taking food from your hand, make a distinctive noise whenever you offer them food – e.g. a whistle or clicking sound.
The mice will associate this sound with food and learn to come to you when you make it.
If I accidentally made a clicking sound near my mouse cages, about 50 little faces would appear at the wire demanding sunflower seeds!