Now that is not easily explained.

Basicly a mesh consists of a list of vertices and a list of indices.

If a polygon/face consists of 3 vertices for the first face you would need to look up the vertices for indice[0], indice[1] and indice[2]. So if indice[0] returns 10 you need to read out vertice[10]. etc.
You should be able to read this info from ID3DXMesh .

I cannot tell how to do this specific for ID3DXMesh as i do not use D3D.