To cut these boards to size, the tool to use is a score and snap knife. Once you have marked the lines you want to be cut you will then place a straight piece of material along the line as a guide. You need to run the knife across the board and along the guide several times to make a groove. You can then snap the board along that line.
You also don't want water getting in under the edges. You can either use a Brutus 4.5mm x 2.4m Building Moulding Division to join the sheets together or seal the gap with Sikaflex 11fc. If you use strip joiners they will stick up proud of the surface and there will be a visible join on the wall. If you are going to paint the wall then it might be worth going with the Sikaflex product and skimming it flat so it is flush with the board’s surface. This will give you a cleaner result. - MitchellMc