you cut the hole, and a piece of plasterboard, into an oval shape so it will go through the hole, but when rotated will block it
you drill a hole into the oval and thread a piece of string through it
you buy the smallest possible pack of Plasterboard Joint Filling Plaster (preferably powder) and mix up a yoghurt-pot full
you butter the edges of the oval with the soft plaster, put the oval through the hole, pull on the string so it presses against the inside surface of the plasterboard wall and hold it for 20 minutes, or wind it round a piece of wood to hold it in place
When set, you mix up some more, and while holding the string, butter it into the hole so it sticks to the edges of the hole and the face of its oval (you are holding the string in case you dislodge the oval).
When this is set, you cut off the string and finish filling the crater.
Do your initial filling with a flexible metal filling knife, but your final with the widest flat-ended filling knife or scraper you can find, or a plasterers trowel if you can find one. The idea is that the blade is wide enough to press on the sound flat plaster on both sides of the hole, and press the filler flat.
Don't overfill it above the level of the wall.
If it needs smoothing, scrape it with a wide metal scraper while still cheesy. Don't sand it.