One characteristic of modernist literature is its focus on the individual. Modernist writers often created characters who were in conflict with a changing world, demonstrating how they adapted to change. Let's first examine how "A Rose for Emily" exemplifies this characteristic.
The story's protagonist is a member of a once-prominent Southern family. However, Miss Emily's modern world views her with scorn instead of honor, and she remains isolated from a world that has moved on without her. Miss Emily intentionally defies the social expectations for her privileged heritage when she attempts to court Homer Barron, who is not only from a lower class but is also a Northerner.
When Homer doesn't seem to be a "marrying man," Miss Emily again takes control of her destiny, poisoning Homer and keeping his corpse locked in her bedroom for years. Miss Emily's macabre attempts to assimilate into her changing world is also an example of absurdism, which is another characteristic of modernism. Unable to adapt to her society's progressive restructuring, she turns to demented and absurd methods to demonstrate her own independence.
"This Is Just To Say" demonstrates modernism through its experimentation with form. The poem has no rhyme and no meter; indeed, it lacks any punctuation whatsoever. Instead, the poem reflects a stream of consciousness that focuses on the speaker's experience instead of any sense of predictability.
This poem also focuses on the individual; the speaker does not apologize for his actions and instead only reiterates the enjoyment he reaped from stealing the plums, which were designated for other purposes. Creating meaningful symbolism is another characteristic of modernist writing; the symbolism of the plums themselves is central to the meaning of the poem. Some believe that the plums might be an allusion to the Biblical Tree of Knowledge, found in Genesis. This poem's focus on human thought is a quality that was common to modernist writers.
I hope this helps as you continue to explore these qualities in some of the other works you reference. I am linking the eNotes guides for these two works below to help you analyze them. Best wishes!