That's my biggest problem.

There are two ways to handle them:
1.) The polygons within a model are sorted by their internal number (time of creation). So the first created polygon will be rendered first. For my flower model (see screenshot) I first modelled the lower part of the flower. So this lower part will always be rendered in the background. And the white flower was created last, so it is always rendered in the foreground. So I have no sorting problem.

2.) The model itself is sorted by it's X,Y,Z-Position. You can change the centre of the model to affect the rendering/sorting.

It is much work to prevent wrong sorting.