This is a large topic. Let's use two parts of Jimmy's thoughts, found in Chapter 1 (originally O'Brien's 1986 short story) of The Things They Carried. We know two of Lieutenant Jimmy Cross's deep thought from the opening lines.
First Lieutenant Jimmy Cross carried letters from a girl named Martha ... Slowly, a bit distracted, he would get up and move among his men, checking the perimeter, then at full dark he would return to his hole and watch the night and wonder [about] Martha ....
Jimmy Cross had deep thoughts about Martha and about his men who were under his care. His thoughts on both were conflicted and somewhat confused. In the one instance, he knew Martha wasn't in love with him though he was certainly convinced he loved her. He read and reread her letters and imagined scenes between them and outing they might have. He carried his hope that he might join her light world as the man of her true affection.
On the other hand, he was concerned for the safety and well being of his men though he had two problems related to this responsibility. The first was that he was distracted by his feelings for and daydreams of Martha. The second was that he burned with guilt over the death of Ted Lavender.
To expand the elements that might comprise points in a monologue, look for and analyze, as above, other thoughts Jimmy Cross expresses. An example of a monologue incorporating these two deep thoughts might run something like what I've composed below:
Now is my hour to nestle with Martha or walk with her along the shore again. "Love, Martha." No. It's not that she loves me. It's just a way of signing. A way everyone signs. I carry hope though. I carry hope that after the war, if I get through this with my men, after the war, she'll really love me. She and her knee from the cinema will really love me. I dreaded seeing that sad look when I reached out and touched her knee. I guess it was too close to the explosive end of Bonnie and Clyde.
Dusk. Time to look after my men. See that the parameters are secure. Lavender. If I hadn't been daydreaming about Martha, if I had been fulfilling my responsibility and focusing on war, Ted would still be alive--all right--Ted might be alive--no, he would be alive. Martha wrote more letters after I burned all those distractions on April 16th, she wrote more, more that I willingly let distract me. My daydreams break my promise to my men. To protect my men? Or to dream of Martha? Parameters all secure. Men all accounted for. Martha. Martha and my thoughts of her through the long cold night.