In order to spin on a roller skates you need to practice standing on two wheels. Then do half a rotation by pushing one leg backwards and the other one forward. In the leg that is pushed backwards the weight should be on the front wheel and in the second leg on the rear wheel. After controlling half a spin, you start working on making a full rotation and then more rotations. You should practice rotation in both directions. During the spins, push your legs closer to each other and keep the hands close to the body, to increase the speed of the rotations.