Anonwelder published an excellent Instructable, showing how to make a safe, wireless-controlled mains switch which could be used with Google Home and Alexa based on the Sonoff wireless switch.
His/her Instructable gave all the details you need to make the switch and cannot be improved upon by me.
I followed the instructions, but adapted them to a 'Poundland' 3-gang extension cable (roughly equivalent to a Dollar Store in the States).
There is no point in going step-by-step through the process again. I just present a series of photos, showing exactly the same device, but based on the £1 extension cable.
The hardest thing may be removing the six screws from the mains socket!
Step 1: A Series of Photos Showing the Process
The six screws holding the unit together have 'anti-tamper' screw-heads with three lobes. These are a pain; happily I have a suitable socket for my screwdriver (otherwise, I would replace them with normal self-tappers for future access).
Like Anonwelder, the pairing button is accessible through one of the original 13A plug holes.
Mains electricity is lethal - if you try this and are not a qualified electrician, do not plug it in until it has be checked by a professional.