First and foremost, it is not really recommended to tinker inside a tube amp unless absolutely necessary, so make better use of other maintenance activities, such as replacing a faulty tube as an excuse for some cleaning. This is a complicated enough process to warrant clearer instructions than we are able to provide here, so make sure to check a number of tube amp guides before setting out.
However, what you should always bear in mind is that the power is turned off before proceeding and, if possible, the electric cord isn’t plugged in an outlet.