Here is what you do
you line up all the letters to make sure they fit on the sheet. Then you remove them one at a time and with a sharpie you write the corresponding letter on the cookie sheet.
Then you have your pre-schooler match the magnetic letters to the ones on the cookie sheet. You tell her she has 10 seconds tops to do it otherwise she doesn't get dinner.
Bonus, you hated those magnetic letters because you didn't really have a good place to keep them (yes, I hate having my fridge full of letters too), well now you can just keep them on the cookie sheet. Other bonus, we use the inside part of the cookie sheet (the blank side) to help Enzo master his spelling words of the week. He gets tired of writing, spelling with magnetic letters is way more fun.
I love using the stuff in my kitchen (old cookie sheets for example, food, food coloring) to help my kids learn. Periodically I'll post a kitchen school project. If you have any you'd like to submit and have featured here at Kitchen Corners send me an email. I'd love to see what kind of stuff you enjoy doing with your kids.
Check out We Can Do All Things for more projects using a cookie sheet.