A literary commentary can be a lot of work write, but it can also be a lot of fun. The reason for that is because a literary commentary allows you to write your opinion on the novel or parts of the novel. The trick is defending your opinion with evidence from the piece of literature.
Step one is to read "A Scandal in Bohemia." I'll assume that you have done that. The trick with your read though is that you have to look for more than plot events. Analyze the tone and mood. Analyze characters. Are they static or dynamic? What are the major themes of the piece? Is there are particular audience that it is written for? Did the author use specific stylistic and/or diction devices that make the piece stand out for some reason? If it helps, make a list of the above items and jot down some notes about each.
Your next step is to plan the argument that your literary commentary is going to make. That's super important. A commentary is going to make an argument. You need to be able to state your argument in a single sentence. I'm sure that your teacher has called this part the thesis statement.
I'm going to assume that even if you haven't read "Romeo and Juliet" that you are familiar enough with the story for this example to make sense. Let's say that I have to do a literary commentary on that play. I've read it, and I will assume that my audience has read it as well, so my commentary should NOT be a plot summary. My argument is going to be about the love and family feud of the play. "Although at first glace 'Romeo and Juliet' appears to be a story of lost love, it is more appropriately a warning to readers about the dangers of pointless feuds."
That thesis is going to be the focus of my entire commentary. I will use specific pieces of evidence to support my argument and rebut any potential counter arguments. Your commentary on "A Scandal in Bohemia" will do the same thing. Figure out what point that you want to make. Then use plot events, character descriptions, and quotes from the text to support your argument. Avoid over summarizing large parts of the story. Lastly, make sure that your conclusion really pushes your reader to care about your point.