I have a model and have sculpted in skin pores. However now, at over 5000000 polys, it is slowly lagging. How do I convert it to a normal map so that I can remove the multires and a replace with a low level supdiv modeifier.
You need to make low poly version of the same object and bake a normal map from hight poly to low poly object, but before you do that it is required that you UV unwrap the low poly one, the high poly is not needed. Here’s the video that wil give you a little bit more insight of the process that I haven not fully described: https://www.youtube.com/watch?v=bGmYwpim6sQ
You can start from here and explore other videos to grasp what is needed for a good normal map baking.
You do not need a second object if you are using multires. You would need one if it were a dynamic topology sculpt. For multires just change the preview subdivision level to something lower than the render and sculpt value. 1 is generally good. Then in the bake panel just make sure you have Bake from Multires checked.