There are a few ways you can do it, but one is...
Get some bonding plaster and fill the channels, leaving it a mill or two below the surrounding wall surface. Let it dry and then fill it with normal all purpose filler (polyfilla). If you've got a plastering trowel - ideal - if not a caulking blade - if not wide filling knife. To make life easier, make sure the chases are free of all dust/debris, and slap a bit of 5;1 unibond in there before applying plaster (Care needed - obviously electric off - hopefully sockets not in place at this stage).
Once filler dry, sand smooth and fill again. Ideally don't paper, just spend time filling and sanding. Never like paper in bathrooms and kitchens unless absolutely necessary

Hope this helps,