How do you convey your idea when you can't convey your idea?
I'm also not great at presenting. One method I've found useful is to present to and get feedback from people who have around the same level of knowledge as your intended audience (i.e. people who are not too heavily involved in your project, but are in your research field). This means they will be able to tell you which parts you need to explain in more detail. 

Also, look at other people's presentations to see what's the standard level of technical detail in your field - I've seen presentations where the focus should be on the scientific analysis and results, but instead too much time is spent on the minutia of how the analysis was performed.