This phase is the first step in realising the project that you roughly sketched out in the idea phase. The preparation phase is often the most time-consuming one, so make sure to allocate enough time, so you will be able to have everything ready for the participation phase.
In sum, the idea phase was all about framing your project in a youth-appealing way and winning the support of decision-makers. Now it is time to concretise your project idea and set it up in detail, so you will be able to achieve your goals.