When scoping out and planning work, an easy way to get a comprehensive understading of everything that needs to happen is to enumerate every scenario. I find this technique saves a lot of time and clarifies what to do in a way that avoids surprises later.
How do you do it?
Start by laying out each variable. For a product feature it might be some aspect of the user and the action. For example: is the user new or existing?, did they take the action yet?, is the action complete or in-progress?, did something go wrong?
This gives you a set of scenarios to enumerate: new user but hasn’t taken the action, a new user who has taken the action but it’s not done yet, a new user who completed the action, a new user that encountered an exception (same for an existing user). Now you can trim down the list of permutations to the ones that makes sense and decide how to handle each one.
That’s a trivial example, but I find the same technique is just as useful when negotiating a contract and wanting to make sure all your bases are covered.