Well, I think it's a vastly long discourse to do on a forum. There may be Youtube video or articles on it, try Google
Basically you need to:
-identify where your deck
attachments are to be
-Design the shape you want, some are tall and rounded some are low and more squared off.
-Most have the forward "hoop" tubing a bit lower than the aft one..
-Most people never fold up their dodgers, but if you want to do this, be sure to design it around having that feature.
-Once the tubing is fabricated and installed, use clear plastic to create the fabric
If by "how to measure" you mean to buy a premade dodger
. Ask the maker what they need.