I'm about six months out of Japanese so I'm a little rusty, but I can try to explain a few things.

ni is always a destination, and in some phrases you have to understand that the literal english translation isn't what the Japanese would use. For example, where we say "take a bath" they say "enter the bath" so the bath is treated as a location. o is for direct objects that you're performing an action on, which you seem to have down.

Wa and Ga are weird particles because they're both technically subject markers. The difference, from what I recall, is placement in the sentence. Wa is always for if you're indicating the subject at the start of the sentence, and ga is for subjects later in the sentence. de is for the location of an action. I think a good way of differentiating between it and ni is like saying "I eat at a restaurant" (something like "resutorando de tabemasu") and "I am going to a restaurant"

Is that helpful at all? It's a little weird. When I had to analyze a long sentence, I would always look for the nouns and verbs I recognized, then try to contextualize to understand the particles. A lot of the way we learned was just through pure repetition of simple phrases using those particles, then expanding on them to more complicated forms and repitition. See if there's anything like that in the book or if your professor could write some example sentences starting from base usage up to more complicated usage with multiple particles.