-
Something like this is a good place to start :-
https://www.decoratingdirect.co.uk/viewprod/r/RODPFB/
I have so many different types of filler knives. It's useful to have something wider than the channel you're filling. You can knock up the filler in an old tub, make sure you follow the instructions and measure the amount of filler to water, then mix it until it's not lumpy. You can use warm water to make it dry quicker. Once it's dried for a few hours you can use a sponge to clean up any excess or texture the surface a bit.
It's not that difficult but you need smallish amounts of a few materials that come in larger quantities.
Hopefully your wiring is in some form of trunking possibly plastic, pinned into place so that it's easy to plaster over.
PVA the channel, mix up some bonding plaster (not multi finish), slap that in and roughly level it off, best if its a little below the surface but just less than proud is best.
Let the bonding plaster dry, 24hrs should do it if the central heating is working. Wash the bonding off the walls surrounding the channel with a sponge. Should now be clean walls.
Use some TX110 from toupret or similar if you have another filler you prefer. Fill the channel to flush. If you want a slight texture to match surrounding walls then get creative with a sponge. Let it dry a bit then sponge of any excess.
Paint with Gardz from Zinsser. Let that dry and finish with emulsion.
So you need pva, plastic trunking and some nails, bonding plaster, decorators filler & gardz