By the way, this is the quickest method to make a low poly version of your model, but is not the exactly right method. It works fine – I have used it in three games so far – but it depends on your game company. Also it is not good for characters where the animator will need good topology on the mesh. It is good for rocks, cliffs etc., but your developer or the engine that you use may not like it and you will have to make a nice, clean and topologically correct mesh for low poly version. There are many ways to do that with ZBrush retopology tools, applications like Topogun – I recommend that – with 3ds Max 2010, graphite tools etc. Anyway it's just something to bear in mind.
Back to the tutorial and it's time to make your UVs. I won't explain how to make them – the process is easy with the Max Pelt option. You can use whatever app you want for that. I'm going to the bake the normal map part directly. You have the low poly mesh with UVs now. Load your high poly mesh again. They have to be like it the ones shown in Fig.10. Then, in your Max modifier list, choose the Projection modifier. Pick your high polygon mesh. Check Shaded in the Display section and then increase the amount in the Push section. You can see the cage around the mesh; you want to increase the amount enough to avoid intersection with the mesh and that's why Shaded is checked.
Now with your low poly model selected open the Render To Texture panel and press 0 on the keyboard. Here check Enable in the Projection mapping section, add your normal map and set the resolution – 2k in my case (Fig.11). Press Render and your normal map is ready. You can delete the high poly mesh now; you don't need it anymore. Save your scene and export the low poly model that has UVs.
Now it's time to go back to ZBrush for some textures. In ZBrush, load your model of the arch and open the SubTool tab. Click the Append button and choose PolyMesh3d (Fig.12). Select the Polymesh subtool and from the Tool palette, click Import. Find your low poly model now with the UVs. You can see how the star-like object will be replaced with the low poly model (Fig.12a).
Divide the new mesh a couple of times and then press the ProjectAll button that is at the bottom of the subtool menu. That should project all the details from the old mesh onto the new one that has the UVs (Fig.13). Then you can delete the old mesh if you want or hide it.