This is very easy.
1. Marinade the salmon for a bit.
2. Heat the cast iron skillet very hot, then add then salmon skin side down. Leave alone for 3 mins or so
3. Take salmon off, peel off skin. Add some more marinade to the de-skinned side, then sear the de-skinned side for a couple minutes.
4. Flip, then sear the other side for a couple more minutes. It’s done when it flakes easily.