For format, check with your HR, they will most likely have a company specific format.
What I think you are really having trouble with is how to compress this to 15 days instead of a longer period. I would suggest that you include a followup meeting at least weekly to go over the progress on the plan. Depending on what you write, you might want a daily meeting or one every couple of days. What you need to do is be very specific about what actions the person needs to take or no longer do.
Discuss with your manager what they want to put into it. If the employee has multiple problems, prioritize fixing them, a person can legitimately only work on so much.
Since the timeline is so compressed, I have the sense that this is really a way to check off all the blocks to fire someone for cause (to the skeptics, yes I have seen PIPs that were used to salvage otherwise good employees not just to justify firing them).
In this case, it is important to write the PIP so that the case for firing is clear if it is not met. It is possible if you make it too vague or include more things than anyone can do, that it could be contested. That is why it is important to do this in consultation with your manager and your HR to make sure that the PIP will legally serve its purpose.