Hi @iyanghian,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
The basic process would be to remove the handrail, ballusters and the framing for the handrail, then frame the opening, sheet it with plasterboard, plaster it, then prime and paint.
While everything is achievable D.I.Y., I wouldn't call it a beginner job because you would have to frame out an irregular opening and then install irregularly shaped plasterboard and finish it against an existing surface. This is not a straightforward task, and if you don't have experience with this kind of construction, it isn't going to be an easy project to learn on.
Unless you were very confident in yourself, I'd suggest you contact a carpenter for their assistance with the framing and a plasterer for the finish work. Painting it yourself would be a way to reduce the cost, but the other tasks are likely best left to the professionals.
Allow me to tag @Dave-1 and @Nailbag for their thoughts.
Jacob
