using RNGCryptoServiceProvider to generate random string

I'm using this code to generate random strings with given lenght

public string RandomString(int length)
        {
            const string valid = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
            StringBuilder res = new StringBuilder();
            Random rnd = new Random();
            while (0 < length--){
                res.Append(valid[rnd.Next(valid.Length)]);
            }
            return res.ToString();
        }

However, I read that RNGCryptoServiceProvideris more secure than Random class. How can I implement RNGCryptoServiceProvider to this function. It should use valid string just like this function.