Before attempting this I had a red-ring-of-death, and now the system isn't even lighting up. I press the power button and nothing happens, no fans, no lights. Not sure where I went wrong, or if there is any way to recover.

I found it much easier to remove the SATA cable after lifting the assembly mostly out of the case.

I found this step difficult. I wasn't able to see where the clips were from the pictures. If you look at step 14 you can see what the clips look like after the vent is taken off. Once you realize where they are it is a lot easier.