Janice: I have done two of them so far and used generic kits I bought in Yuma from a fellow who repairs them for a living. They're a fiddly project for sure and impossible
to write all of the steps needed to get it right but can give you some tips.
If you're a fulltimer use the bed as your work surface, if at home use the floor. This project can overwhelm a large table.
Remove all of the end caps and carefully separate top and bottom (they slide apart) sections (Day & Night) into separate groups laid out as they came apart. Carefully note the route the cording follows from the upper tension spring down the night, dark section and then across the middle bar and then down the day, light section and back across the bottom bar & out to the bottom anchor. Do not cut the old cord as you will need it to establish correct length for the new stuff.
Make a wire loop out of single strand wire of the thickness of a paper clip and have the loop about 6 or 8 inches long so you can thread this loop through the collapsed blind sections, put the new cording in the loop and then pull it through the blind.
The trick is to leave extra cording so that when finished, you can tie off to the anchor having the top tension spring stretched just enough so that the blind will stay up but not so tight that the cording is trying to saw it's way through the blind material.
The wider blinds over the dining table or back window have four runs of cording
I have not attempted one of those yet.