Pitch - describe the basic functionality of the app in one paragraph of less
Define - List the features/scenarios the app will support
Sketch - Draw a very basic wireframe sketch of the main "screens" of the app
Expand - Build a comprehensive spec document, leaving out steps or requirements will make it difficult to plan effectively and will likely force major changes or cuts later.
Plan - Based on the feature list and spec, create a full development plan making sure to keep an eye on the total amount of time required and to include buffer for things going wrong. Be sure to prioritize tasks so that cuts can be made if necessary.
Start Coding - once you have your plan written begin to implement the project according to your plan.