You can make a masterwork item: a weapon, suit of armor, shield, or tool that conveys a bonus on its use through its exceptional craftsmanship, not through being magical. To create a masterwork item, you create the masterwork component as if it were a separate item in addition to the standard item. The masterwork component has its own price (300 gp for a weapon or 150 gp for a suit of armor or a shield) and a Craft DC of 20. Once both the standard component and the masterwork component are completed, the masterwork item is finished. Note: The cost you pay for the masterwork component is one-third of the given amount, just as it is for the cost in raw materials.
Making something masterwork doesn't make the base item take any longer or cost any more, but it gives you an additional 150/300 gp item that you have to craft after the base item is done before the item is useable. So it increases the time by however long it takes you to make something masterwork, but has no relation to the item you're working on (aside from whether it's a weapon or anything else)
There was a discussion I was having with my GM (and I say discussion as we weren’t arguing about it) that at first I felt the masterwork component could be created at a later time as it requires its own roll and has its own money value.
But my GM indicated that is not the case as the masterwork component is part of the weapon, armor, item, etc. and cannot be made separately at a later time and therefore you can’t just ‘plug’ it in, to put loosely.
The rules don’t really specify how the masterwork component interacts with the item, so it’s not really clear. But I can certainly see my GM’s viewpoint and it is of course his call.
The whole reason the discussion came up originally was because another party member wanted to make a masterwork bow, but really didn’t have the skill to make the masterwork component at this time, and I suggested to just make the bow now and craft the masterwork component at a later date as it has its own GP value and own roll to make. But that is when our GM had come in with his ruling.