How to fill holes in plasterboard wall left by old single plug sockets.

Clean loose material. Lightly spray the hole to dampen. Use a spray foam and fill the void then leave to dry. Cut back the excess so it's just below the surface of the existing plasterboard. Use finger if needed to push it back a little. Get some ready mixed filler or powdered whichever you prefer and fill the dent bringing it flush to the existing wall. Leave to dry and sand excess smooth. Then paint or paper over. That's how I do it as it ensures a solid crack resistant finish and saves faffing about trying to cut bits of plasterboard to the right size. Save so much messing about and gives the best finish possible. In my opinion.

Buy yourself a Half sheet of 9mm plasterboard. Fix with timber Batons. Scrim tape gaps, PVA glue and plaster fading the plaster out. Or for a non plaster fill with easy fill instead of plastering. Best way is to have them plastered.

Best thing to do is get a plaster to do it

If you get some board adhesive and some plaster board cut all your pieces out first that they are as tight as possible within 2mm around mix your adhesive AFTER boards are cut then apply a good dab in the hole to back of board and squeeze into level with wall leave for a day the you could use paper tape and some easy skim to blend the joints out or polyfilla and sand depending on how much your budget stretches on this part and finish required for yourself.