start by reading carefully the 8257 datasheet
try to get a good copy of the datasheet since they were all scanned from old books, or old copies.
try to undersatnd exactly the functionallity of the part
what are inputs and outputs, and were the are connected to.
what the timing relashionship between signals.
what are the differnt operation modes reuired, and what
is the purpuse of the various registers.
then try to create your block diagram of the differnt modules your project will use.
you can use simillar block diagrams as specified i the datasheet or invent you own.
then try to implement each block seperatly and test it (simulate).
finally integrate all blocks to one project and simulate it by using simulation models of peripheral and memory devices.
after simulation succedd try to syntisize your project, fix errors, and then try to implement a bit file.
then you can check if it is working on the FPGA.