To become a certified plasterer, the tradie would have been under apprenticeship for several years and earned a Certificate III in Solid Plastering. The total time depends on the employer and exact type of certification they want to achieve.
The intended surface needs to be prepped before plaster can be applied. Remove anything like wallpaper and poor paint with a scraper or sandpaper. PVA or a textured tape can also be put on the surface to increase the surface area for plaster to bind to.
The best plaster depends on the intended purpose. Bonding plaster should be used on smooth surfaces, hard wall plaster is a great base coat, and tough coat plaster should be put on last because it is the most durable .
Plaster shaped in decorative ways is called carved plaster, or plasterwork. For decorations, a more durable plaster is used to cope with being shaped at odd angles and being more exposed to bumps and weathering.