In general, if you use the sheet-metal module, you have the option, through the appropriate command, to flatten the model. that I know is the only way to do it, clearly if someone has a better idea to help jasmtn... .
This geometry is not directly developable, you must first break it into developable elements (the two cylinders and the cone), then make tears along a generator and then you can proceed to development (total or piece).
I suggest, however, to start making the component as a combination of the three small cylinder pieces, cone trunk and large cylinder. in this way you will be able to do everything easier, even because in reality the particular I think you will have to do it just like that.