Use a business closing and sign your name. Above all, be professional. If the you’re sending a physical letter, print it out and sign it by hand. Otherwise, just typing your name will do.
The most effective letters of recommendation are written by professors or work supervisors who know you well enough to describe your academic, personal, or professional achievements and potential with candor, detail, and objectivity. Letters that compare you to your academic peers are often the most useful. Most schools do not consider general, unreservedly praiseworthy letters helpful.