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We use google drawings and low fidelity (grey rectangles devoid of detail) Sketch wireframes for this step, but feel free to use anything. It usually takes the form of a flowchart 
ie:
Volunteer clicks signup →
Volunteer selects a signup method (facebook, github, linkedin etc) →
Volunteer confirms account details →
Account created
Step 4:
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Interface design
How will the layout work? How will the devs start building it?
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