Softplastics are lures that are, well made of plastic. You insert a Jig head into them to get weight and action. Softplastics are very versatile as you can use them for small fish like Kawhai or big fish like snapper and maybe even knigfish. Color and shape matters alot.
To insert a Jig head into the softplastic, first put the jig head side by side with the plastic and see where the hook would come out from. Then insert it hook first horizontally into the softplastic, making sure you get to the area where the hook will protrude. Then simply get the hook through the top of the plastic and make sure the top is sticking out from the head of the plastic.
The size of the Jig head you should use depends on the size of the softplastic you are using and the fish you are targeting. Generally, a jig head that is about the same size as the softplastic is a good starting point. But usually you want to use a 1/8th oz size jig head for smaller softplastics that target Kahawhai and around 1/6 or 1/4th oz for larger plastics that are the size of your palm.
The color of the softplastic you should use depends on the water conditions and the fish you are targeting. In clear water, natural colors like brown, green, and black work well. In murky water, brighter colors like chartreuse, pink, and white are more effective. It's always a good idea to have a variety of colors on hand and experiment to see what works best in your local waters. Personally, i use white softplastics since where i fish Whitebait are very abundant and the softplastics mimic a whitebait which gets me alot of fish.