Locate the beam close to the area where the leak is coming from. Identify the area of the leak by the looking for the stain in the ceiling. Once you have found the beam in the leak area, look for the other beam in a drier area of the ceiling right across from the beam you first located.
Cut the beam in a squared shape using a knife, preferably a utility knife. Focus on the central part of the beam that you selected in Step 1. For more help in making a precise cut, use a straight-edge tool to draw the square.
Measure the length of the dry beams you want to cover. Make the hole big enough to take out the wet plaster you will find once you go in. Now you are ready to cut the drywall. Put the drywall flat on the floor that has big enough space for you to move around it. Use the measurement that you took of the dry beam to cut the drywall.
Put the drywall up on the ceiling and use screws and drywall tape to put it in place. Now use the spackle knife to apply spackle compound around the edges. Wait for 24 hours for it to dry.
Sand the area smoothly. If necessary, apply more spackle compound and leave to dry for eight hours.