Widescreen Pixel Aspect Ratio Step-by-Step

I have read a number of the posts dealing with pixel aspect ratio when dealing with footage shot as 16:9 widescreeen. I may be a bit dense, but am having a bit of trouble understanding the steps. Does anyone have a step-by-step tutorial so that I may sucessfully do the following:
1. Export 16:9 (1.2 pixel) from After Effects into PI to use as a BG.
2. Line up and key frame my PI effects to match the video BG I imported from AE.
3. Remove BG.
4. Save Output the footage from PI so that I can import into AE.
5. Specifically, what do I need to do in AE so that all the work I did in PI to match the effects with my BG footage and key fram the effects to very specific points, still matches up exactly when imported into AE.
I am working in NTSC. A simple step-by-step would be much appreciated. Although I am very comfortable with AE, and reasonbly comfortable with PI, please don't leave out any steps as each one is likely crucial. Thanks!

Lyon Films LLC