A lot depends on the fish species and size. This works well on bass, pan fish, etc.
Get a sharp knife with a narrow, long flexible blade. (Electric knives work just fine)
Without scaling fish, place flat on cutting board, remove head.
Start cut holding knife parallel to cutting board just above head end and follow top of backbone. Plan to cut through rib bones and entrails.
Place other hand flat on fish to hold in position and "saw" knife blade through rib bones (can be difficult depending on size and type of fish. Why an electric knife id a good choice)
Be careful to follow backbone, don't cut through it, (or your hand) back to tail - stop cutting before reaching the tail and don't cut through skin.
Flip fish over to scale side and use same "sawing" motion to remove both skin and scales.
Take fillet and with a curving motion remove rib cage, along with entrails. Follow curve of rib bones. Remove any fin bones in a similar manner to produce a skinless, boneless fillet. Repeat on the other side of the fish.