Okay , this is a question that has come up a lot and I'm sure it will pop up again.
I had the same question when I saved some spoonbill skins for backing bows and spent a lot of time searching back posts on this subject .
I'm not familure with thinner skinned fish like carp but what I figured out will work with spoonbill and large catfish .
After I did a lot of searching I came up with this .
On a spoonbill I split the fish down the center of the back and belly giving me two larger skins .
After splitting the fish I fellet knife between the skin and meat being careful not to cut the skin .
After I get the skins free I place them on a smooth surface and get a large knife and scrape any meat left on the skins off .
After I get the meat off I go back and scrape the skins down hard to break up the oil pockets and force as muck oil out as possible , I also shave down any thicker places on the skin to keep any oil from being trapped in those places .
After scrapeing the skins down good I mix a stong bucket of dawn dish soap an water to soak the skins in , after soaking the skins for an hour or two in a cool place I scub the skins down with a brush then rinse them in clean water .
After rinsing the skins you can place them in a freezer bag and freeze them for later or stretch them out on a sheet of plyboard to dry , when stretching place the tacks or small nails close together to keep any wrinkles from forming , like rawhide , as the skins dry they shrink a lot pulling nails over or tearing loose , another reason for using a lot of nails .
After drying I cut the skins into strips a little over two inches wide for bows and role them up and place them in freezer bags and keep them in the freezer .
When I'm ready to use them I take them out and wash them with acetone to get any surface oils off and sand the flesh side to rough them up and remove any thick spots .
After I sand them I soak them in warm water and dawn soap until they are soft enough to work with then I rinse them off and pat them dry with a towel before I glue them on .
This is how I worked with my skins , if you have any other ways feel free to share , this might save someone a lot of searching later .
