Mistletoe can be successfully grown from berries but this is by no means easy and it may take many attempts before you succeed.
To increase your chances of success it is best to collect the mistletoe berries in February or early March not at Christmas! You may need to cover a few berries on an existing plant with a net or you run the risk that birds will have eaten them leaving none for you to collect! The seeds will stick to the bark of the trees when the berry is smeared onto a branch. In the wild this is done by mistle thrushes and other birds which are fond of eating the white mistletoe berries but discard the sticky seed by wiping their bills on a convenient branch in the process spreading mistletoe from tree to tree. Mistletoe will only grow on certain tree species and apple trees seem especially receptive. Hawthorn, lime, poplar, whitebeam, pear, field maple and ash are also suitable trees to try.
Chose a young branch with thin, smooth bark and wipe the seed in a shady position such as the north facing side or underside of your chosen branch. There is no need to cut the bark with a knife. You will need patience. It can often take a couple of years before the first pair of mistletoe leaves appear. Prior to that you may if you look carefully spot the fragile green root which is the first sign that your berry has germinated successfully. Remember while mistletoe does not kill trees it is partially parasitic so you may not want to plant it on your best fruit tree! Once it's established it is fine to harvest some each year and both the mistletoe and the host tree should survive for decades.