Safety is always first, make sure the plug sockets are off before starting to work around them.
Measure the distance from the skirting to the bottom of the socket.
Lay the MDF over the top of the socket and mark in pencil where the MDF meets each side of the socket.
Take the MDF off the wall and lie it flat on the floor with the pencil marks facing up. Then use a straight edge to continue you marks down the panel.
Using the measurement from Step 2, measure up from the bottom on the panel and mark with a pencil. Use a spirit level to draw a straight line across that measurement so you get a H shape.
Optional: Use a drill and a large drill bit to drill holes in the corners of the cut out (pink circles on the diagram below). This will help with sawing them out.
Use a hacksaw to cut out the section for the socket.
Place your panel on the wall to check the fit. Use sandpaper for small adjustments or the hacksaw for bigger ones until you get a snug fit.
Don't worry about any gaps, you can decorators caulk to fill them as per your kit how to guide to get a straight finish.