Qualified person arrives at house and generally pokes about to see what level of wall / loft insulation you have, whether windows are DG, what method of heating the house has. Does a little calculation of his PC-thingy, it throws out a rating in the bands A to G (?), A being best.
Produces a smart little pictorial akin to the things you see on fridge doors in Currys - coloured arrows. A good house will come out at B or C.
EPC assessment is part of having a HIP done - most people get it done by appointing a HIP organisation who will subcontract the EPC to a specialist person.