I recommend the Gun Digest series of books on firearms assembly and disassembly. Amazon, among others, sells the individual books (Part V deals with shotguns).
Search on youtube for Winchester 1300 assembly, they are internally the same.
You need to take it to a competent gunmsith
Best left to a trained gunsmith
This will not fit a model 12
Your winchester model 12 shotgun was made by winchester in 1928.
Your winchester model 12 shotgun was made by winchester in 1950.
Your winchester model 12 shotgun was made by winchester in 1956.
Your winchester model 12 shotgun was made by winchester in 1939.
Your model 12 Winchester shotgun was made by Winchester in 1926.
your Winchester model 12 was made in 1970.
Your winchester model 12 shotgun was made by winchester in 1937.
Your winchester model 12 shotgun was made by winchester in 1947.