Being able to use scissors is a skill that develops slowly through childhood and children will develop this skill at different ages depending on their developmental stage, motivation for the task and opportunity for practice.
Most children will learn through practice and visual and verbal prompts, while practicing alone may be enough for some. Reminders to keep 'thumbs up' can help improve their grasp and will make it easier to snip the paper. If a child needs additional support consider modifying the task to ensure success, you can do this by providing scissors with shorter blades which are easier to manipulate, use thin card rather than paper as this is easier to hold, cut larger sheets of paper into quarters, and by starting with thicker lines which gradually get narrower. Using stickers or dots can help to prompt turning corners and changing direction.
Here you can find more advice on developing scissor skills at home and at school. If a child continues to experience difficulty controlling regular scissors, try spring scissors or if the child has great difficulty using scissors there are a range of adaptive scissors available.