Plaster cracks are caused by a variety of reasons. Some of the common causes are due to seasonal moisture changes, wind, foundation movement, lumber shrinkage and clay soil movement. If you can determine the source of what is cracking the plaster walls, it is best to fix that first before repairing the actual plaster so the repairs will last longer.
Remove all loose plaster from the wall being repaired.
Use a triangular can opener or a sharp metal object to enlarge the cracks that you will be repairing. The top of the crack should be at least 1/4 inch wide. If the crack is already larger than that dimension, widen it a bit further to get it ready for the patching compound.
Clean all dirt, debris, grease and dust off the plaster wall that is being repaired using a moist rag. Make sure the wall is completely dry prior to repairing so you can closely inspect exactly where the cracks are located.
Read the directions on the bag or box of powdered joint compound to see the manufacturers recommendation of how much water to add into the powder to create a putty consistency.
Dampen the area that will be patched using a moist rag prior to applying the patching compound.
Use a broad knife to apply joint compound to fill all cracks. The technique is similar to icing a cake. It may take a bit of practice to get the hang of smoothing out the walls with a knife.
Cut strips of fibreglass mesh or paper tape that will bridge any wide gaps between cracks or for persistent cracks. Either of these methods will strengthen the bond of the joint compound to the wall and help resist shocks to the wall if the wall should move after it is repaired.
Fill in the wide crack halfway with joint compound.
Press the fibreglass mesh tape or paper tape into the wet joint compound if the cracks are wide or persistent. Apply a second coating of joint compound on top of the tape or mesh after the first layer has completely dried. Apply a third coat to even out the surface.
Allow the wall to dry completely. Lightly sand the wall to create a flat, smooth finish. Use a damp sponge to wipe off any dust or residue.
Some joint compounds contain higher amounts of gypsum and tend to harden within 30 minutes. If you prefer a longer working time, you can buy mixes that take up to two hours to harden. If the hole you are patching is deeper than 1/2 inch, fill the hole only halfway with joint compound and allow it to dry. Then, repeat the process to fill in the remainder of the crack. Do this even if the crack is wide and needs to be bridged with paper tape or fibreglass mesh. Joint compound can be smoothed out with a damp sponge as it's drying. Professionals will use water and a trowel to achieve a smooth, glasslike appearance, but this takes a lot of practice. Make sure your knives have blades that are wider than the width of the hole you are repairing so you can get a clean sweep across the crack without creating indentations in the wet joint compound.