It is rather a complex business learning to use scissors. Children often use 2 hands at first.....scissors have 2 handles afterall!
However, firstly a child needs to learn to pick the scissors up and organise their fingers and grip. It's useful to place the scissors flat on the table with the small hole towards the child, so that the shape of their right hand naturally leads them to put their thumb in the small hole and other fingers in the other hole. Its best if the other hole is large enough for 2 fingers. (index and middle). If it is a one finger hole then the middle finger needs to go in, and the index finger needs to be positioned forward of the hole. This gives the best stability and control.
So the child has hold of the scissors. With their thumb uppermost. It's great to present a strip of paper holding it for the child, because then the magic can happen. The child can do one close of the scissors and cut right across so that a piece of paper falls off. This is the magic motivator. If the scissors are spring-loaded, then it is even easier. The first step is learning to close the scissors to cut and this situation creates success. After a bit of practise, the child will probably want to hold the paper with the other hand.
Using scissors means some motor planning and sequencing is needed. After closing, the scissors need to open so another cut can be made. Some children may quickly pick this up. Others really benefit from spring-loaded scissors.
A strip of light cardboard can lead to the child being able to apply a bit of pressure, and the learning that different materials require a different amount of pressure. Cutting straws is fun because pieces bing away, and require even more pressure which strengthens the muscles in the hand.
Further development involves learning to open and close while cutting across paper or light card. Just after the open a tiny draw back is needed to reposition the scissors for the next cut. Learning to stop at a point requires some finesse, as the end point cannot usually be seen when nearing the end. Do you snip with the point of the scissors on the end point, or do you cut nearer the inside of the blades for more control. Following a line requires judgement and alignment.
And then the big thing is turning the paper with the other hand so that a curve or a corner can be cut. There is a lot of co-ordination between the 2 hands going on.
Using scissors requires planning, organising and coordination. When observing children learning to use scissors, it can be seen just how much integration of many steps is going on. Do make sure that the scissors are sharp enough to cut. It's frustrating when the paper just folds between the blades!!! Not encouraging at all.