Starting is easy; starting in a direction you can finish is harder. I try to make the first session about clarity, not polish.
Name the scope
Write one sentence: what is allowed to be in v1, and what is not. The “not” list saves weeks later when something shiny shows up and tries to bloat the idea.
One constraint
Pick a single constraint: time, platform, or audience. A constraint forces decision-making. Without it, everything stays optional and nothing gets cut.
Default to shippable
Ask “what is the smallest thing someone could use?” and aim there first. Momentum is easier to build on a small, real artifact than on a big plan.