When you boot looped did the kindle fire logo turn blue?
The first link should work fine for installing twrp the issue with 7.3 is the boot loader and the entire reason you install the freedom boot part.
It should install twrp and at the same time most likely boot loop your device.
Just try using the key combo if twrp doesn't automatically load.
After installing twrp you need to already have a rom on your sd card and to immediately install it.
I would recommend to try the first link again.
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