If it is a unique Mastercraft without a recipe, then you can just gather the different things together and "trade" for the unique item.
If it is a crafting recipe, you would do a Mastercraft submission for one part, then use a second Mastercraft submission utilizing that part to finish it out. So two months. This came up pretty infrequently, and my guess would be when people wanted to hide the crafting recipe from people they were distributing the finished product to, which would be much harder to do now.