Writing the initials of the names of the three kings in between the numbers of the date (so this year 20+ C + M + B + 11) on the mantel over the entrances to your house is a traditional Epiphanytide custom, usually coupled with the blessing of the house using some of that superstrength Epiphany water. And it's one of those customs with a double meaning - as well as standing for Caspar, Melchior and Balthasar, the letters also stand for 'Christus mansionem benedicat', or 'May Christ bless the house.'
There are actually two options to consider here:
- ideally, ask your priest to come and bless your house! This is a traditional annual visit, and in these days when priests are unlikely (for whatever reason) to just turn up on your doorstep without an invitation, worth considering if he is the willing sort (and not too overloaded with work!); or
- do the blessing of your house yourself. In this case, you can get the words of the full blessing and instructions from the ever helpful Fisheaters website.