One of the easiest ways to add unique details to your project is through the use of option parts. Option parts are a generic term for accessories ranging from thrusters, hatches, and other details to extra and unique weapons. Kotobukiya makes the most options parts under their Modeling Support Goods (M.S.G.) line (HLJ / Hobby Search). Bandai has their own line called Builder’s Parts (HLJ / Gundam Planet).
Option kits featuring weapons or stands come with instructions and can be built like any other kit. Kotobukiya’s weapons generally come with standard attach points that allow them to be easily utilized with their original models. The fun stuff is the random patches, panels, rivets, thrusters, spikes, lenses and others armament details. These can be glued to your kit in any number of ways to really change how a kit looks. Where you add them, how many you add, it’s all up to you. Painting is generally recommended but if your kit already has a bunch of gray components you might be able to add some option parts without having to paint.