I am new to rooting and tablets. I cannot understand how to load "flash in TWRP". I try running adb commands in CMD but it says it is a unrecognized command. Can someone make this idiot proof? I am doing something wrong and I don't want to brick my kindle hd 8.9
To "flash in TWRP" you need to install a second bootloader and TWRP. The instructions as to how to do so are in this thread - the thread you just posted in regarding returning to stock.
Once you have TWRP installed, you can flash the zip file from within TWRP