In order to make a model, you first have to get a basic idea about the structure or process involved. You decide to make a model to show the differences in size of arteries, veins, and capillaries. First, read about these structures. All three are hollow tubes. Arteries are round and thick.
Veins are flat and have thinner walls than arteries.
Capillaries are small. Now, decide what you can use for your model. Common materials are often best and cheapest to work with when making models. Different kinds and sizes of pasta might work for these models. Different sizes of rubber tubing might do just as well.