Knowing the purpose of the prototype will help you to make key decisions during the prototyping process. At this point, you should have a clear idea of the following aspects:

• The goal: What do you need to learn with the prototype?
• The audience: Who do you need to learn from?
• The scenario: Which activity in a specific context will the prototype support?

This information will help you to decide on a prototyping approach and the tools to use.

Be specific when defining the goal for the prototype. You can decide to learn about the general understanding of your concept, the clarity of navigation, the discoverability of a specific activity, the fluency when completing a task, and many other aspects that contribute to the user experience. It is okay to have multiple goals, but avoid generic ones such as whether or not "the idea works". Otherwise, it won't be clear which aspects are more useful to prototype in order to answer your open questions.

You need to help your audience provide you with a useful answer. Consider whether the selected group of people will be able to answer your question and how the prototype can help them to do so. If you are creating a prototype to discuss concepts with other designers, a low-fidelity prototype may be enough, whereas a high fidelity prototype may be more appropriate when you need to observe specific details of real user behavior.

The prototype should be based on a context that is familiar to the audience in order to help them get into the situation. If you are prototyping a travel app, it may be better to use a well-known location as an example rather than a hard to pronounce, obscure destination--unless that is precisely what you want to test.

You also need to consider how to deliver your prototype to your audience. Depending on the size and location of your audience, you may provide them with a device prepared with the prototype, or share the prototype with them to use it on their own device. Many prototyping tools export to HTML, which makes it ideal for use in many kinds of devices thanks to the widespread availability of web browsers. However, other tools create prototypes for specific platforms, limiting the number of users you can reach and requiring some kind of installation process.

Based on the selected scenario, you can identify aspects of different relevance--the minimum core aspects to support the scenario, those needed for additional context, and those you don't need to prototype. Focus your efforts on the most important parts.

In some cases, it may not be clear what to prototype. You may not be sure about the most promising direction to take with your design. In those cases, you can create multiple prototypes to learn how your users react to different ideas. Due to the nature of time, given time to prototype, you will have to choose between building a more detailed prototype or several prototypes with a lower level of detail.