The plastic gusset plate is made of wooden keel. First, drill holes on the surrounding walls with an impact drill, and install the Yanbian keel, and then install the main keel in the middle; The main keel spacing is about 80 cm, and it is connected with the top surface (with suspenders). The levelness of the main keel can be adjusted according to the surrounding keels. This process requires the use of construction lines, which can be raised a little (about one centimeter) in the middle.
After the keel is installed, you can install the PVC Yanbian keel and then install the gusset plate.
The above is the basic practice, but for a person who has never done it, I am afraid it is still a bit difficult to complete these tasks. So, here's a suggestion: get a carpenter to do it for you.