This is a very basic tutorial that shows how to bake a normal map from a mesh that has a Multiresolution Modifier applied to it. Screenshots are from Blender 2.81.
Start Blender, start a new general file. Remove the default cube and add a plane.
Go to the modifier properties en click on “Add Modifier”.
Select the Multiresolution Modifier
Select “Simple” and click 6 times on “Subdivide”. “Preview” should now have a value of 6.
Go to sculpting mode (top menu) and left click a few times on the plane to create some geometry.
Add a new material to the plane. Name it anything you like.
Go to UV Editing. By default your plane should have a square UV map. If you have a different mesh, make sure you have a workable UV-map.
Click the shading top menu to open the shading window. Add the following:
– Texture Coordinate (Add>Input>Texture Coordinate)
– Image Texture (Add > Texture > Image Texture)
Connect UV to Vector as in the image below.
In the Image Texture, click “New”. Change the name “untitled” to something you like (e.g. NormalMap).
Next, in the Render Properties, set the Render Engine to Cycles. Set the render value to 512. Lower is possible, but you’ll get a better normal map when setting this higher.
Still in the Render Properties, open the “Bake” Settings. Check “Bake from Mutires”, and set the Bake Type to “Normals”
Still in the shading window, at the left side select your image (you probably have to resize the window). Note that in the example below “Normal_Map” is the name I gave to the new image texture. Here is your brand new normal map!
In this same image editor window, click the hamburger button, select image and “save as”. Save your normal map to a location of your choice.
For the sake of simplicity in this tutorial you will just delete the multires modifier. Normally you will keep it and make a copy of your mesh, remove the modifier from that copy – or lower the subdivisions – and continue working on the copy.
Go to the Modifier Properties and remove the modifier.
Your plane will now look like …. a plane again.
Go to the shading tab. In the image texture, set color-space to none-color. Als add a normal-map (Add>Vector>Normal Map). Connect Color to Color, Normal to Normal, as in the image below.
Now, to check if this is really working, set the Render Engine to Eevee.
Click on the light in your scene, and change it to a sun. Move the light to x=0, y=0, z=0.4. set the rotation to x = 125, y=-20, z=260. These are just some values that worked for me.
Go to the layout window and set the viewport shading to rendered.
Now select the light, hit the R key and slightly rotate the light to see the Normal Map doing its magical work!
Final Note. In this case we completely removed the Multires Modifier. Normally you would not do that, you would decrease the subdivision count to one where the polycount is acceptable.