Skin the fish, remove as much meat as possible, lay the remaining carcass in a plastic tray of some kind with many small holes drilled in the bottom, and bury it. After a few months dig it up, wash it off bleach the bones, and there you have it.
OR ... skin, de-meat it, and put in a covered container with water ... plain water ... and let it rot or "macerate" in the water for a few weeks. Open container, hold your breath, wash the bones, bleach, dry and there you have it.
Last option - and the most costly - send the fish carcass to Skulls Unlimited!

It all depends on your budget for such things.
Last,
last option ... buy one from Carolina Biological Supply Company!
http://www.carolina.com/product/life+science/anatomical+models+and+skeletons/animal+models+and+skeletons/fish+skeleton.do?sortby=ourPicks#
Natural bone. Fully articulated and mounted on Plexiglas® plate in sturdy, double-view display case. With illustrated key. Size, 9 1/2 × 5 3/4".
Item # Product Price
243400 Fish Skeleton $176.00
Good luck to you!
John.