Hi Bombelen, The most vulnerable part of the plant that's likely to be damaged by frost is the newly developing fronds in the early spring, to protect them you can scrunch up a ball of hay or bracken (not too tight, mind!) and insert it into the crown to provide some good insullation, It can be removed when all danger of frost has passed, usually late may is a safe bet.
I'ts important to remember the trunk should be kept moist, as some of the root is in there, having said that, you dont want to trap moisture in otherwise rot can set in, so choose something to lag the trunk that can "breath" like horticultral fleece or some hessian sacking etc, Not plastic or buble wrap. It may be possible to erect a small screen around it too, using 4 stakes and some open weak fabric.
I've assumed your fern is outside and not in a pot.
Good Luck Tbird+