Nail wooden or steel battens(20mm – 30mm thick) into the wall and ceiling, with all the corners firstly, then on the plane with the distance of 400mm away between every wooden batten.
Screw PVC profiles(cornices) on all the corners, then insert the panel into the corner profiles, then the folling panel into the first panel.
Screw PVC second panel’s notch side into the wooden or steel battens, then insert the third panel, then the fourth, and nail the fourth’s notch side, then fifth and sixth, then…
Attention for the distance and notch side panel: for the ways above is based on 250mm or 300mm wide panels, and wooden or steel battens between distance is 400mm, so every 2 panel, screw the second notch. Surely you could change the ways as per your situation presently, by way of changing the distance of the panels and the wooden or stell battens.