How shall I report an error in a given paper?
The same TA pointed out flaws (of fundamental nature, for example F = mv instead of F = ma) in the model, and once corrected allowed me to obtain the results shown in the paper.


The fact that after correcting the model you can actually reproduce the reported results seems to suggest that the authors did implement their model correctly, but made a mistake when writing the paper.

I think that it is best to first contact the authors, politely explain what you found, and give them the chance to take an appropriate action, e.g. publish an errata in the journal where they published their article (or on their website, if the mistake was really minor).

If they do not react at all, you might contact the journal directly. The editor will typically take care that an errata is published.

When nobody is responsive (I do not expect that in this case, but with a conference article or a fishy journal that might happen), and you consider it important that the scientific community is aware of the mistakes, you could decide to publish your correction somewhere (journal, conference, website, ...).