The Develop stage is all about developing a frame with several ideas into a fully grown and tested concept. This takes at least one full iteration, which includes doing more research, ideation, visualizing, and reflection. 

To develop a concept, all kinds of things must be developed. The problem is that each of these things is related to other things, but you cannot design everything at once. So, these things must be designed (somewhat separately) whilst keeping in mind how they relate to all other aspects. This will ultimately result in a synthesized whole.

Inexperienced designers consider this to be a (very) difficult stage because there is 'so much to decide on', and 'everything is related'. Good ways to advance are:

• Show & share sessions
  Develop individual ideas concurrently and somewhat loosely connected. That is: develop an individual idea relatively on its own and do not pay too much attention to other ideas. Whether these are ideas that others are developing or ideas that you yourself will develop at another moment in time. Still, you regularly bring everything together in a 'show & share' session so that others know what you are doing, and you know what they are doing. Do not wait until everything is final. Reflect on the work, both on the individual contributions as well as on the results as a whole, and decide together on how to proceed.
  
  
• Set up a 'design war room'
  A war room is a place where all relevant information on the developing concept is put together and is presented by means of sticky notes with ideas on a wall. It enables you to get a feel of the bigger picture while working on individual parts. Often, show & share sessions are hosted in such rooms.

Pixar Animation Studio - Courtesy of Pixar

Learn more

There is an interesting blog on war rooms, including information on how to set one up.