Place the salmon in a deep dish or tray. Pour soju over the salmon, making sure all sides are well coated. Set aside for 15 minutes. Pat the salmon dry with paper towels then set it aside.
In a small mixing bowl, combine the salt and sugar. Note that the salt and sugar should be 15% of the total weight of salmon each. To calculate this, multiply .15 by the weight of salmon in grams. Mix the salt and sugar until well combined.
Place a long piece of plastic wrap on top of your cutting board or work surface area. The plastic wrap should be 3 times the length of the salmon.
Pour half of the salt and sugar mixture on top of the plastic wrap. Spread it into an even layer roughly the same size as the salmon.
Place it skin side down on top of the salt and sugar mixture. Pour the remaining salt and sugar on top of the salmon and spread it out evenly so it covers the entire salmon.
Carefully wrap the salmon, making sure all sides are coated in the salt and sugar mixture. Wrap the salmon a few more times with plastic wrap to ensure it's securely wrapped.
Transfer the wrapped salmon into a large Ziploc bag. Place it on a plate or tray then weigh the salmon down by stacking another plate or tray on top with heavy objects.
Place in the fridge to cure for 24 to 48 hours. Keep in mind that the longer it cures, the saltier the salmon will be. I find that 24 hours is just right.
Remove the plastic wrap and rinse the salt and sugar mixture with cold water. Pat the salmon dry with a papertowel.
Thinly slice the salmon and serve!
