Step-by-Step Guide:

1. Clean and Prep the Hole

• Clean the area around the hole: Remove any loose plaster or drywall around the hole. You can use a utility knife or a drywall saw to clean up jagged edges.
• Widen the hole if necessary: If the hole is large or irregular, you may need to widen it slightly to make a clean, square shape. This will help the patch fit better.
• Remove debris: Make sure the hole is clear of dust, dirt, or loose pieces. Wipe the edges with a cloth.

2. Install a Backer Board (for larger holes)

• If the hole is larger than 2-5 centimetres in diameter, you’ll need to add a backer board for support.
• Cut a piece of scrap wood that’s a little longer than the hole.
• Insert the wood behind the hole and secure it in place by driving screws through the plasterboard into the wood on both sides. This gives you something to attach the patch to.

3. Apply the Patch

• For small holes (less than 2 – 5 centimetres), you can use spackle or plaster patch to fill the hole directly.
• For larger holes, use a plasterboard patch:
  
  • If using a patch, apply a thin layer of joint compound around the edges of the hole.
  • Fit the patch into the hole and press it in place. The patch should fit snugly, leaving only a small gap between the patch and the plasterboard.

4. Fill the Hole with Joint Compound

• Spread a layer of joint compound over the patch (and the surrounding edges) using a putty knife. Smooth it out so it’s even with the surrounding wall.
• For larger holes, you might need to apply multiple coats of joint compound. Allow each coat to dry completely before applying the next.
• Feather the edges of the compound so that it gradually blends with the wall, making it less noticeable when you finish.

5. Sand the Area

• Once the compound is dry, sand it smooth using medium-grit sandpaper. Start with a coarse grit and finish with fine-grit paper for a smooth finish.
• Be careful not to sand too aggressively or you could damage the surrounding wall.

6. Apply Additional Coats (if necessary)

• If the patch still isn’t flush with the wall, apply additional thin layers of joint compound, sanding between each layer. This helps to create a smooth, seamless finish.

7. Prime the Patch

• Once the patched area is completely dry and smooth, apply a coat of primer to the patched area. This helps to seal the compound and prepare it for painting.
• Allow the primer to dry fully before moving to the next step.

8. Paint the Patch

• Paint over the patch to match the surrounding wall colour. You might need two coats of paint, depending on the colour and the thickness of the paint you’re using.

9. Finish (Optional)

• Once the paint is dry, you can apply a thin line of paintable caulk around the edges of the patch for a cleaner finish, especially if the patch is in a high-traffic area.

Tips:

• Be patient with drying times between layers.
• If you’re dealing with a hole in a ceiling, the same steps apply, but you may need to use a stronger patch or different tools to work overhead.
• For very large holes or significant damage, it might be easier to replace the whole section of drywall.

With some patience and attention to detail, you’ll be able to patch your plasterboard and make it look seamless.