How do I develop the cashflow in order to do the NPV and IRR calculations? I struggle for hours with this and feel really stupid.

Cash flows for NPV and IRR calculations are computed as follows:
Operating profit or EBIT
+ Depreciation and amortization
+ Non cash items. e.g. gain or loss on disposal

It depends a whole heck of a lot on what the problem is giving you, and could be very simple and could be very complicated. We would actually have to see the entire problem to know. Since we aren't here to do your homework for you, it would also be extremely useful to see the work that you have already attempted on the problem, and then help you from there.