As a teacher of older children in the primary range I have always been amazed by the weird and wonderful ways that children write the numbers from 0 to 9, even though they will all have been taught how to do this at school, either in reception or year 1.
A simple rule – start at the top of the number. These three maths worksheets help with this. Start at the black dot and follow the direction of the arrow. Trace over the grey numbers with a pencil. Most children can write much more neatly with a good quality pencil than a pen because the pencil provides a much better contact on the paper and is less likely to slip or slide.
The first page goes from 0 to 5, the second from 6 to 9 and then a page showing all 10. Left handers might find the zero harder to do and will be tempted to go clockwise rather than anticlockwise. Try to avoid this as they will also probably write the letter o in the same way and make it difficult to join in a neat style later.
You might want to print these out several times so that plenty of practice can be given. Then see if the numbers can be written on plain paper.