You have no idea where or how to insert the needle, and would most likely end up harming the fish badly.
I think you should euthanize the fish if it's been suffering for this long. Take a bag or a small container, add tank water, the fish and put it in the freezer. Do not open the freezer to check, just leave it for a few hours and the fish will fall asleep peacefully.
Next time you notice dropsy, separate the fish immediately, add a heater to the tank and add some maracyn 2 or equivalent and hope for the best. Don't wait several months before you start feeding them peas