So I do an external build first right?
It's not strictly necessary but it's an easy way to make sure everything works before you assemble it into a case. It's good for a first time build, but if you're confident and careful you can just start assembling into the case so you don't have to later.
Do I need to attach a monitor or can I just listen for a beep w/ the speaker?
While you can listen for a beep, attaching a monitor is still better so you can see in case it says something. However this isn't even 100% necessary. What I suggest though is hooking everything up and going ahead with installing Windows just to make sure it all functions beyond just POST and BIOS.
If I don't need a monitor can I wait on installing the video card?
If you don't plan to use the monitor you can skip the video card for now. If you install into the case off the bat then just install the card and monitor because you're going to eventually.
For the external build should I mount the H100 backplate first before installing the cpu? Does it matter how I orient the H100?
Install the CPU first, because once you do that you can pretty much leave it there, though it doesn't really matter. Just don't fiddle with the board without either the CPU in the socket or the protector in the socket.
Does it matter what ram slots I occupy? I checked the manual on my mobo and it doesn't say anything about that.
If you have one stick of RAM, use the blue slot closest to the CPU socket (DIMM_A2). For two sticks use the two blue slots (DIMM_A2 and DIMM_B2). For four sticks it's obvious
The H100 backplate has a layer of plastic on it, do i remove that or is it there for a reason?
Leave it. It prevents your board from shorting out.