Start children with the alphabet and practicing shapes when writing. Before handing them a pencil try some of these ideas:
Use sand in a container to have them draw with their fingers the shape of letters.
Have them shape letters using clay or Play-doh to strengthen their hand muscles.
Provide a writing tool, like a crayon, that will allow for a good grip.
Allow children to freely make lines and circles that mimic writing.
Give children verbal motivation and exaggerated approval.
Give them a workbook or paper with wide enough lines to practice staying in the margins.
Give children resources that allow them to trace letters.
Let them explore and teach them letter recognition.