What is the best way to go about replacing both headlight units?

Here is a guide that will help with a little bit more general procedures on how to replace these. Clearly the instructions from back then were not terribly detailed so this should help.

https://www.2carpros.com/articles/headlight-lens-replacement

Also here is a video that is put out by a parts website that shows step by step on a sedan but they are the same as a convertible which is what it looks like you have.

https://youtu.be/rvdPfAohIRI

Also, I am not sure why you are replacing them but the common reasons are the lens is broken or they are faded and the lenses are cloudy. If they are cloudy then here is a video that will show how to just fix the ones that you have rather then replacing them.

https://youtu.be/Y_cYcwbB5qM

Let us know if you need more info and how you make out. Thanks