Don't stop at Villard Notre Dame, carry on and climb the Solude.
Ignore the private road signs (the locals are friendly).
Gravel Warning - it's very gravelly (but still fine on a road bike).
Near the top there is great views of Alp D'Huez, and the top is signified by the smallest of col signs.
Over the top and the road surface returns to tarmac and is good for the descent.
The road then joins the Col d'Ornon 6km from the top.