You must farm 2,000 grand company seals (from fates, turn ins or Grand Company Leves) and then buy the issuance from the Grand Company quartermaster.

It should be listed in the "tools" tab (I don't remember if it's called tools but it will be the one that isn't "weapons" or "armor")

You then take the issuance to the chocobo porter in the city your grand company is located in.