The UF2 bootloader is neat in that it exposes a USB filesystem that you can simply drag-and-drop files into from your host OS. Here are some links to resources for building and burning:

Building

https://www.hackster.io/wallarug/uf2-bootloader-creating-custom-boards-c9620c

Burning

Using Atmel Studio (Windows Only)
https://learn.adafruit.com/how-to-program-samd-bootloaders?view=allow-to-program-samd-bootloaders?view=all

Using Command Line (Hopefully Linux, too)

https://forums.adafruit.com/viewtopic.php?f=57&t=142170&p=707151#p707151