I already knew how to fillet a fish. To turn it into sashimi, you just go a few steps farther: you lay your fillet skin side down on your cutting board, and you take off its skin with your willow-blade knife. Then you cut out the spines from the middle of the fillet-- that is, you cut each of the 2 fillets into 2 smaller boneless fillets. You feel them carefully to make sure you got all the bones, and if there are any remaining, you cut them out. Then, assuming you started with a sashimi-grade fish, all you need to do is slice these fillets, and you have sashimi!
I'm so happy that I can do this now!