There are several approaches that can be taken here given the assignment as posted which seems somewhat ambiguous. For one thing, writing about something can involve mere description; on the other hand, writing about something can also imply that an analysis should be included.
If there should be an analysis, perhaps you can take the approach of examining the motivations that lie beneath the actions. First, a perusal of the chapters involved with the narration of the incident will be helpful for your understanding as well as for supporting details. Also, a close reading of the dialogues of those involved will also provide some insight into your analysis.
The most obvious conclusion that a reader can reach regarding Bob Ewell's testimony is that he does not wish to have any of the members of the white community believe that his daughter or he are what was termed then as a "n--- lover." For, although he is a drunkard and an abusive and neglectful parent and at the very bottom of his society, Ewell does not wish to fall into the above-mentioned category; since as a white man in the Jim Crow South, he is still above the good Tom Robinson. And, it is because he does not want to be perceived as condoning his daughter's actions, he legally attacks Robinson and physically abuses his daughter for possibly bringing this unspeakable "shame" onto their family.
Of course, Mayella changes her attitude toward her involvement with Tom Robinson out of fear of her father. Nonetheless, even though Bob Ewell is clearly a reprobate, Mayella's conduct is even more reprehensible as she has been the initiator of the action as well as the recipient of the charity and kindness of the harmless Tom Robinson.
If you choose, then, to examine the motivations of the characters, your paragraph will essentially be a character analysis. So, be sure to support your points by citing the words of the major characters involved. Good luck!