place portal if properly shaped procedure help

Hello, I am trying to create a portal for a custom dimension using the "place portal if properly shaped" procedure because I don't want to use the standard portal igniter and I instead want to use a custom fluid block to light the portal. The only problem with this is that nobody told me how to use this procedure, it doesn't include a place for me to put blocks, the igniter block, or the shape I want it to be in, there are no tutorials on youtube, mcreator, or google and I can't seem to figure it out.