Do i need to assemble it? How tedious or difficult is it
Cole will assemble it for you
But gluing rubbers is not very complicated, it'sa bit intimidating at first but it just takes a bit of practice.
You don't even need fancy table tennis brand name glue, you can use waterbased latex glues like Tearmender (or copydex) which you can often find at your local hardware stores, or you can even use VOC based rubber cement or weaker contact cements, such as Elmers.
Waterbased glue is a bit more difficult to spread if you don't have the right kind of sponges, but if you have aquarium media filtering sponge with semifine pores it is usable. You can also use things like a business card, or even your finger to spread the glue, but this is more demanding and time consuming (still doable). Rubber cement is easier to spread thinly and quickly because they usually have brushes built in to the bottle.
Put a layer on the face of the blade, and a layer or two on your rubber. With waterbased let each individual layer dry for about 15-30min or at least until the layer is transparent and no longer liquid. Rubber cement dries pretty quickly but it will tend to expand the sponge of the rubber a bit, this will usually die down in a few hours. Line up the rubber on your blade at the top of the handle, and gently roll the rubber onto your blade with some sort of roller (be careful not to stretch the rubber with too much pressure)
You can cut using scissors or a utility knife/scalpel style blade. I have had bad success with utility knives, scissors are more reliable for me, but there are guides out there for using different blades. If you lubricant the scissor blades with a bit of water or oil they will cut a lot smoother. Make small cuts and take your time for a smooth result.
I have glued and cut my own rubbers for longer than a decade now so I am very comfortable doing it, but it's also valid to want it done before the complete product reaches your hands