Internal brackets first,
then attach upper and lower external brackets to the bumperette,
then attached upper bracket to body (internal bracket),
then attach lower bracket to the boot floor underside (internal bracket).

Do this sequence make sense?

Sounds fine, but the real answer is what ever works for you.

Jack

I'm continually amazed at how important the installation sequencing is on these car. I spend way too much time installing, removing and then reinstalling (some times repeating the process). I disconnected my clutch and brake lines two or three times last night just to get them and their clips to align properly in order to connect to the blanking plate.