I've done this recently, I followed(ish) [1], and [2] recipes, but modified them for buster.

I'd write it up, but I'm way to lazy^Wbusy right now.

[1] https://markuta.com/how-to-build-a-mips-qemu-image-on-debian...

[2] https://www.aurel32.net/info/debian_mips_qemu.php