Sending a demand letter is easy. Since it's not a required part of the mechanics lien or any other recovery process, a claimant can deliver it by hand, send it via email, or mail it to the intended recipients. But, in terms of "filing" - a demand letter shouldn't typically be filed in the county record like a mechanics lien would.
For more help on sending demand letters, here's an excellent article: Construction Demand Letters – How To Write One That Gets You Paid.