How to generate an `Expression` reference to a method parameter

If I have a method that is building code using Expression trees, to handle runtime types, how can I make an Expression that references a parameter to the method?

E.g. in the below code, how do I build Expressions to pass in that reference the method parameters?

public static bool ExpressionContains(string s, string sub) {
    var cmi = typeof(String).GetMethod("Contains", new[] { typeof(string) });
    var body = Expression.Call(cmi, s ???, sub ???);

    return Expression.Lambda<Func<bool>>(body).Compile().Invoke();
}