Stay away from the light, no wait go INTO the light

That part always confused me, too. I can think of only one reason.
Although Diane specifically commanded her daughter to avoid the light, Carol Anne was still under the influence of "the Beast." Tangina was instructing the entrapped souls to "cross over," and Carol Anne was supposed to direct them to the light, but not enter it herself. Steve must have assumed Carol Anne was following them into the light, so he panicked, and prematurely pulled the rope.

If Carol Anne went into the light, she could never return because she'd have crossed over. Her mother was saying whatever she could come up with in order to keep her away from it. When the time was right, Tangina did her "exorcism", beckoning/luring the lost souls into the light in order for them to cross over. The rope is used to pull Carol Anne back before she goes all the way into the light. The intention was never for her to go all the way in.

Yeah that was annoyingly unclear, whatever their intent. Just a contrived way to ramp up drama and yelling and conflict. I’ll never forgive you for this!

Zelda could have just told them her plan.

The Ghostbusters woman was just a hack. Her light advice wasn't worth the paper it was written on...

Otherwise it seemed pretty straightforward - you need to go towards the light but only when the timing was right.

It's a matter of no one being sure of what the light is until Tangina appears, fzane. Carol Anne is afraid of it because she senses the connection between the spot and the ghosts. When she communicates this, her parents and Dr. Lesh assume that the location is a threat. The picture that Ryan draws displays that he has the right idea about what is happening, and it surprises me that he doesn't mention it. The clairvoyant dwarf verifies that the light is heaven and says that the spirits will follow whom they have abducted. If they can finally get to where they belong, then the beast will lose his power and no dead person will bother the Freelings again. Tangina is aware of precisely where Carol Anne is, so Diane can tell her daughter to stop at the right time. The little girl is told to not look at the light so that she is not mesmerized by it while waiting outside of it. If that happens, then she won't respond to her mother and the monster might be able to overtake her. This scene brings a small controversy because Diane obviously believes that her child will be claimed by the great beyond if the latter steps into its beginning, but the action itself should be impossible. The fascination with Carol Anne in the first place is that she exudes a powerful state of being alive. She is still a human body, and heaven does not accept bodies. It is not clear whether Tangina thinks the same thing that Diane does or if she just wants rapid access to the missing person.