* the second one for the persistant environment (size is up to you, at least 256k)
* the third one for the kernel (2 MiB ... 4 MiB in size)
* the 4th one for the root filesystem which can fill the rest of the available space
* Mark the first partition with the partition ID "53" and copy the bootlets into this partition (currently not part of @b barebox!).
* Copy the default @b barebox environment into the second partition (no filesystem required).
* Copy the kernel into the third partition (no filesystem required).
* Create the root filesystem in the 4th partition. You may copy an image into this partition or you can do it in the classic way: mkfs on it, mount it and copy all required data and programs into it.