HTC EVO 4G :: Current System Back Up And Installing Custom ROM

I've read through the step-by-step here, and while I've found it to be the clearest set of instructions I've encountered, I still have a couple points of confusion. It sounds like by the time I get to the end of Section 2 (version 1) I will have a custom ROM installed (2Q). Now I've used Titanium Backup (I used the pre-OTA Unrevoked for su) to back up apps/data/settings. But I'm not sure what that gets me. What if I want to revert the phone to exactly how it is now? A Titanium Restore would restore apps/data/settings into the custom ROM loaded in 2Q correct?

It appears my answer is in Section 3, item 3D (NAND backup). But at what point do I do Section 3? If I've already gone through Section 2, my current ROM is overwritten by the custom ROM loaded in 2Q, so it's too late to back it up, right? Perhaps I'm making this more difficult than it needs to be, and I'm usually not a complete idiot about technology! But I'm having trouble getting my arms around some of these concepts. I see a lot of instructions saying "do this, do this, do this", but very little explanation what's actually happening when I do these things, and why I'm doing them, so I'm failing to grasp how the various pieces and actions are fitting together.