string cookieValue = Guid.NewGuid().ToString();
//Creating a cookie which has the name "UserId"
HttpCookie userIdCookie = new HttpCookie("userId");
userIdCookie.Value = cookieValue;
//This is where you would state how long you would want the cookie on the client. In your instance 2 days later.
userIdCookie.Expires = DateTime.Now.AddDays(3);
Response.SetCookie(userIdCookie);