Walk around the room and find the lowercase letter cards. Mark an X on the matching capital letter on the chart. Use the color you see on the card. Send it to the teacher.
I hang lowercase letters around the room, such as these (free): https://www.teacherspayteachers.com/Product/Free-Alphabet-Letter-Cards-1828806 . The letters are in various colors, so students can change the pen color to match what they find. You could also have the kids write the letter, but my students are not ready for that yet. Have fun!