When I taught my son how to use an adult-size toilet, I had him sit backwards (facing the tank). He could lean forward and use his two hands to hold onto the seat to stabilize himself. This helped him feel secure instead of feeling like he was going to fall into a giant toilet! The only downside to this technique is that they have to take their pants all the way off- rather then just letting their pants fall to their ankles.
Another trick using this style of sitting is allowing your child to draw with a dry-erase marker while sitting on the toilet. This can help add an element of fun which can help take the pressure off learning a new toilet technique. It can also help if your child has a hard time sitting still long enough to go. After they are done, have them wipe the lid clean with a wet wipe or wet cloth.