To download an image using the UUU tool is simple, you just need to set your board into serial download mode and then run the following command at the windows shell terminal.
.\uuu_imx_android_flash.bat -f imx8mm -a -e
Remember that you need to have the uuu.exe file in the same path that you have all your images. Furthermore, that command will only download the image into an sdcard of 16 GB. If you want to use the eMMC memory of the board, you need to change the settings of the script. For more information regarding this, I suggest you follow the Android's_Quick_Start_Guide. There you could find in a more complete way how to use UUU tool.