Patch repairing plaster can be difficult, hopefully these instructions will make it much easier.
Often when you remove wall paper from a wall or decide to decorate you will find that a small amount of plaster will fall away exposing the brick work behind. This is quite normal but before repair can begin it is a good idea to tap the surrounding plaster with your knuckles, if the plaster sounds hollow then this too is loose and needs removing!
Here you can see that a small section of plaster has fallen away from the wall. Test the surrounding area with your knuckles and if it sounds hollow then remove this plaster also.
Using a bolster chisel and club hammer remove all of the damaged plaster and square off the hole as in this picture. Remove all plaster back to the brickwork and thoroughly clean the bricks to remove any dust or debris.
Apply some PVA bonding to the bricks and allow to dry.
You can either repair the hole by using a piece of plasterboard or by using 2 coats plaster.Plasterboard - cut a piece of plasterboard 9.5mm thick, slightly smaller than the size of the hole that you have made and fix it to the wall using a few small dabs of dry wall adhesive making sure that the plasterboard is sunk back at least 1mm from the plaster.
Plaster - Mix enough backing plaster (either bonding or browning) for the area and apply using a hawk and float, push the plaster in firmly until it is level with the old plaster.
If you have used the plaster method then the plaster now needs cutting back 1mm to allow for the coat of skimming plaster, this can be done with a float but I prefer to cut a piece of wood like the one below-prefer to cut a piece of wood like this
Scratch the back coat with a nail or similar to help the finishing coat adhere to it.
You will now have to wait for a couple of hours whilst the plaster dries a little, if leaving it for a day or so be sure to wet the backing coat thoroughly before applying the finishing plaster.
Mix the required amount of finishing plaster and apply it using the hawk and float, the plaster should be rested on the hawk and transferred to the wall using the float. The plaster must be mixed to the correct consistency so don't make it too runny and ensure there are no lumps. The key to successful plastering is holding the float on such an angle that it is nearly flush with the wall but not quite, this takes practice and if the float falls flat then it will remove plaster! Polish the plaster by throwing water on it from a small brush and trowel it out until it is smooth. A polished surface is easy to achieve with practice.