The gulbi will be frozen. So let it thaw out at room temperature. (To speed up the thawing process, you can also soak the gulbi in a bowl with cold water). It should take 15-20 minutes.
Once the gulbi has thawed, use a pair of kitchen scissors to trim-off the fins and tails off the fish (reference video). Then use a knife to carefully de-scale the fish. Remember to get both sides, underside and top-side.
Now thoroughly rinse the fish off under running water.
Afterwards, pat the fish down dry with a few paper towels - remove as much moisture as you can.
Then take out a small bowl and mix vegetable oil with vinegar. Set aside for next steps.