Reverse engineer it, design hardware, write software, test it, get the bugs out and you're home free.
OBD1 is not a protocol. It only requires the manufacturers to produce a lot of paper work (usually full of mistakes). The connector and the method of communicating (protocol) is at least different for each and every manufacturer. Sometimes manufacturers use several of these protocols.
Which car do you wish to use it on? Make, model, model year, engine type, fuel used etc. etc.