What I've done so far is nothing new. I've just applied already existing theory and numerical codes to specific cases...
I don't know what subject you are in but that is new. You have taken an already known theory and applied it to something else, even if that is a specific case of something. I am assuming you got results from applying this case, in which talk about those lots in your evaluation.
which should be easy to do for people that already understands the topic and has technical expertise with the numerical code I'm using
Which means it was probably overlooked. Do not assume that if something is easy to do and no one has done it that it is not important. There could be many reasons why they chose to follow a different path of research, you cannot second guess why.
I am a first year PhD in CS, I have struggled many times that what I am doing would be easy, simple, obvious to others to do. One thing that keeps me going is this quote: "standing on the shoulders of giants".
My view on the report; include the following two aspects:
- Your contributions, why is it different, if you have applied it to specific cases say that as that is what makes it unique.
- Evaluation. This is where you can prove you are different, that even though you "only" took a theory and applied it elsewhere you can show it worked (or maybe didn't).
In all good luck with your conference and poster, and enjoy it!