Economic models give you a framework for thinking so you can logically and systematically analyse changes in different aspects of the economy. Typically building an economic model starts with simplifying assumptions: Assumptions for the Production Possibility Curve (PPC):
i) The economy can only produce 2 goods: Whch 2 goods is often decided by what you want to illustrate or analyse so for example for an economy it could be capital goods and consumer goods (quick bit of thinking - what do these terms mean?)
ii) It is a closed economy (no international trade)
iii) Factors of production are not perfectly interchangeable (some are better at producing Consumer Goods and some are better at Capital Goods)

Diagram:
More often than not for economics at the level of IB models are centred around a diagram and showing you know how the diagram is constructed is a key skill.
Take a piece of paper (Exercise book or A4)
Draw 2 axes (not too small so it is clear) and label them Consumer Goods (X-axis) and capital Goods (Y-axis)
Reason like this:
If the economy were to devote all its resources to producing Consumer Goods there would be a maximum it could produce. Plot this point somewhere along the Consumer Goods axis.
Alternatively if the economy were to devote all its resources to producing Capital Goods there would be a maximum it could produce. Plot this point somewhere along the Capital Goods axis.
Now plot all the points that show all the combinations of Consumer Goods and Capital Goods the economy could produce (take note of assumption iii above and this will determine the shape of the PPC.
Can you work out how the PPC will be drawn? Draw the curve.
Plot a point inside the PPC. What can you deduce if production is at this point?
Plot a point beyond the PPC and what can you categorically (without contradiction) say about this point?