Note that everybody is really doing the same thing as in an ordinary Spin the Top from a wave. The center and end dancers trade places by turning with whichever hand they are holding, and then the end dancers walk 1/4 around the center of their wave -- which in this case is also the center of the whole square. The reason that the center dancers turn 3/4 as a star, rather than an ordinary two-dancer arm turn, is that there are four of them doing the action at once in the center.
As in an ordinary Spin the Top, everybody ends up holding hands with the same person with whom they started. Many dancers use this fact to decide how far to turn. They simply walk forward until they meet that same person. Another way that some people approach this is to count the dancers they are passing. The fourth person that they meet will be the person with whom they were originally.
All 8 Spin the Top can also be done when dancers are facing in a circle, as they would be after an Allemande Left or a Right and Left Grand. In this case, they start by stepping to right hands, then continue with the rest of the action.
This case is also basically the same as an ordinary Spin the Top when done from facing couples. Everybody steps forward to make a wave, and then does the call as if they had already been holding right hands.