Ok lemme get to things straight, this is the kfhd forum, if you have a kindle fire 1 then this is technically the wrong forum but i will point you in the right direction becasue theres some things you need to understand. First off the kindle fire 1 is very very easy to do what you want to do to it. Basically you download kindle fire utility
and it will root and install whatever recovery you want onto it and from there you grab rom's from the kindle fire 1's development section
and you can flash them with the corresponding gapps.
Here's where the important thing is, if you want to do this to a hd model it isnt the same process, if you attempt to use kindle fire utility on a hd model you are likely to hard brick it because the hd has a locked bootloader. The hd has a simple enough method to flash it though, as long its not the 2013 model you can install roms on it, theres a tutorial for installing with fireflash that i think is the easiest method as long as you make sure to downgrade the bootloader, somehow people tend to miss the checkbox for it.
If you mention which hd you have i cna link you to the tutorial for it but i don't wanna send you the wrong one even though there really isnt much of a difference since its the same program and inlcudes the files in teh apk.
Now to answer some questions:
First off the backup in twrp is useful yes, but only if you have twrp installed, lets say something happens and you no longer can access recovery and are forced to use fastboot, in that case you would need the image files that you mentioned, those images can be downloaded off the internet or you can jsut backup up your kindle and use those images.
Second: The wipe options are in twrp once you have it working
Third: You don't copy the boot.img over, you hit install in twrp and select the zip file that is the rom and tell it to flash.
Fourth: do as said in previous step, just do it after you flash teh rom though.
and finally yes its kinda recommended to have a fastboot cable, though i am one to talk since i don't have one but i have the means to make one at any time. Hope this helps!
Also thought i'd mention if it turned out to be a kindle fire 2 that its process is similar to the hd's so dont use kindle fire utility on it either. You can always check software version that it has installed an i can tell you for sure what model it is.