The first step is to download the bootloader from github: https://github.com/StratifyLabs/Nucleo-F446ZE/releases. The bootloader is installed using the mbed USB connector. You just drop it on the mounted disk.
Second, you will need to install Stratify Link which allows you to install the kernel and HelloWorld directly from an online repository. It can be downloaded from: https://stratifylabs.co/download/.
Then you follow the steps show in this video. The bootloader, OS, and application can all be installed in less than a minute.
Once the bootloader is installed, you will need to connect the second USB cable with connects to the STM32 native USB port.