Hi @Ammy007,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
Firstly, I just want to confirm that the height measurements of 5.8m and 4.2m are correct.
If so, then this is quite a tall wall, which would mean a scaffold would be 100% necessary for safety reasons. Working at this kind of height adds significant risks to the project and there is very little margin for error when you are working at those sorts of heights. If you have never done a project like this before, I would seriously consider having a trained professional do it for you, as this is not a great situation for you to be learning.
In saying this, the timber used in your images is called DAR pine, and it is likely a similar profile to this 92 x 11mm Pine FJ Primed DAR. The second image looks to be using two different thicknesses of the same profile, so this 92 x 18mm Pine FJ Primed DAR is another option that could be used in conjunction.
To cut them to size, it would be best to use a mitre saw, but you could get away with using a circular saw with a speed square, or a hand saw with a mitre box.
You would then need a way to mark out plumb and level lines to ensure everything lines up well and looks good. This could be done with a long spirit level, but a cross-line laser level would be the best tool for this.
The timber mouldings would then be attached to the wall using a combination of trim nails, shot with a brad nailer, and a construction adhesive such as Liquid Nails.
Once the timbers are attached to the wall, you would then fill all the nail holes with a filler such as UNi-PRO Multi Purpose Interior Filler and caulk the seams with Selleys No More Gaps Interior Multipurpose Gap Filler before priming and painting.
Check out How to Install D.I.Y. Wainscoting for an example of a similar kind of project.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
