How to cut a brick wall opening
Grab your level and place it against the edge of the wall. Check if there are any gaps between the wall and the level. If there are gaps, you will need to adjust the external angle to make sure the wall is straight.
Position the external angle at least 10mm off the ground. This way if there is any movement on the floor the angle will not crack. Place the external angle against the straightest point of the corner, roughly in the centre. Position this firmly by placing pressure onto the middle of the angle, and pin into place with your staple gun. To check the angle is straight all the way down, place your straight edge alongside the angle, adding pressure. Once this is straight, staple the remainder of the angle into place.
You can also pin the external angle into place using a hammer. Hold the external angle up to the corner, using your straight edge to determine if the angle is straight. Add pressure with your hand to the opposite side of the angle to hold this into place. Then place a nail through a circle on the outer edge of the external angle and hammer into the plasterboard. Repeat this in multiple spots along the angle until it's secure.
Now that your external angles are in position you can mix up your plaster for the base coat. In this case we have two externals, so we need to fill the bucket to at least three quarters to have enough. Once the plaster is mixed to the right consistency, you are ready to apply it to the wall.
Use your hawk and trowel to apply the plaster to the external corner. Start applying to the middle of the wall first, working downwards and taking any excess off the edges. When you reach the bottom, run the trowel to the top of the wall to smooth this out, making sure you have gone passed the architrave line. Then apply plaster from the top down to complete this side. Now start on the other side of the corner, applying the base coat from the bottom up.
Once the first coat has dried, smooth the surface with your scraper to get it ready for the second coat. Apply the second coat of plaster to one side of the corner only and make it slightly wider than the first. This coat should be at least three quarters of the length of your trowel. Use the same processes for applying as the first coat. Leave the face side of the corner at this stage.
Now that you have applied a top coat to one side, you can apply a coat to the face using a slightly different technique. This time when you put the mud on, hold your trowel at a 45-degree angle away from the side that has already been troweled.
The process for applying the top coat is the same as the second coat. This time make the surface you apply slightly wider. Once the top coat is dry, you can sand it back and you are ready to paint.