Demo video: https://youtu.be/Dn869EoTvH4
Hey, I have been working a lot with cube maps recently. They are easy to make, but a bit hard to set them up to be converted in the right way. With this file you can do it very fast.
A good cube map resolution is 3072-2048. You can have bigger and smaller images as long you keep it proportional (1536-1024 ; 768-512, etc).
You just need to:
Load an HDRi; Preview it (optional); Bake it; Save it;
HDRi on the example image is from: http://www.hdrlabs.com/sibl/archive.html