Before you are going to install the tiles, Clean and free of loose paint on the surface.
Plan ahead the layout of the panels on the wall.
Using a level you may draw the panels on the wall before installing them.
Apply glue on wall and panel
First install the full panels, do all the cutting at the end.
Prime panels.
Use the sandable drywall compound to fill the gaps between panels and sand them smooth (use fine sandpaper - 220 and up and do not apply too much pressure on the panels once sanding)
Apply coat of primer over the joints and paint the panels .