Option 1: Long soak: Rinse off excess salt and then soak for 12-24 hours in room temperature water. Replace water at least 3 times.
Option 2: Quick method: Rinse off excess salt and then bring the cod to boil in a pot of fresh water. As soon as it starts boiling, drain off the water and repeat. Do it a total of 3 times then taste the cod to see if it's sufficiently desalted. If not repeat once more.
Make and shape the croquettes
Peel the potatoes then slice them into thin medallions (this helps them cook faster). Put them in a pot with water and boil until fork tender (10 minutes) then drain them and mash - they don't have to be mashed very smooth.
Dice the chives and mash the garlic.
Crumble the cod into a large bowl (the cod should be soft enough to just fall apart).
Add all the other ingredients to the bowl and mix it well until the mixture is homogenous. It helps to use your hands.
Shape the dough into approximately 18 oval shapes. Set aside until ready to fry.
Fry the croquettes
Heat the oil to 150 C / 300 F.
Shallow fry each croquette for about 3 minutes per side or until golden.
Put fried croquettes on a wire rack to drain.
Notes
* Whether you add salt depends on how much salt you draw out of the salt cod. I recommend mixing up the croquette dough (except the egg) and tasting it to decide if it needs more salt (the cod is cured and not raw anymore).