Buy paper with lines that are spaced about 1 inch apart. Preschoolers who are just learning to write won't be able to write small letters, so they need to have plenty of room to form each letter in between the lines. If you can't find paper with wide enough lines, you can draw your own lines onto paper using a ruler and pen.
Create a space at the top of each paper for children to write their names. Preschoolers should be able to at least recognize the letters of their names if not write them, so this space will give children additional practice with this skill. You may simply write a heading reading "Name:" followed by a line. Make the line long enough for children to write their first and last names.
Fill the lines of one sheet with the letters of the alphabet, leaving a bit of space between letters. Use dotted or dashed lines. You may use either lower- or upper-case letters. Children will learn to form the letters by tracing over your lines.
Make another sheet that focuses on only one letter. This gives children the chance to concentrate on one letter at a time. Write the letter over and over on the first line, using dotted or dashed lines. Leave the rest of the lines blank so children can form the letter on their own without tracing it.
Make a third practice sheet to teach children how to write lower- and upper-case letters properly. Draw a dashed line down the middle of each empty line. Use dotted lines to write both the lower- and upper-case versions of one letter onto the sheet.
Photocopy all the sheets. Make about a half dozen copies of each sheet per student. You may use these sheets during group writing time, but you might also keep extra copies on a low shelf so students can practice writing during free time.