Suppose we have five rigid bars whose lengths are 110, 125, 200, 300 and 380 mm. Take four bars to make a 4-bar mechanism which satisfies the Grashof's law. Draw your mechanism and pick one from the four bars as the ground so that the mechanism shows the crack-rocker motion.

In a four-bar Mechanism, the Grashof's law gives a relation between the four bars for which, if it is not satisfied, the mechanism can't move freely in a full revolution.

If we split the mechanism in 2 chains of 2 pieces, in this case: L₁ and L₂ for one side and L₃ and L₄ for another side, the relation should satisfy:

L₁+L₂<L₃+L₄

Where L₁ is the bar that is steady in the ground.

In this particular case:

380mm+110mm<300mm+200mm

490mm<500mm

This selection satisfies Grashof's law.

The crack-rocker motion is done when one of the bars connected with the ground can make a 360º rotating motion, while the other bar connected with the ground made an incomplete rotating motion (can't make a complete circle and act as a pivoting bar). To make this kind of movement you only have to ensure that the bar picked as rotating piece (L₂) is smaller than the piece picked as pivoting bar (L₄).

The image of the mechanism is attached to this answer.