This is the actual start of project. Following things needs to executed in this step.

1. Design a data model based on requirement analysis. It could be star schema or snowflake schema based on requirement. Data modeling consist of following three parts.
a. Conceptual data model
b. Logical data model
c. Physical data model
2. Document in the data model design process including all possible details.
3. Get it reviewed and cross check the data model with reporting needs and your technical team.
5. Design a ETL process. ETL can be done using home grown tool or commercial tools. Having commercial tool speed up the process and also make is easy to maintain as skill set and support is available easily.
6. Create a sample data for the project. this could be a snap shot of the OLTP data.
5. Create a Unit testing plan and test ETL and data model for every new addition and change.
6. Try creating sample reports on top of it to test it.

Many times this step can be skipped if Organization is ready to implement the project and clear about why they need it. Following points can help you to successful execution of this step.

1. Get the most talented and experience people in this step as there past experience in designing  will help a lot to design a prototype fast.
2. Make sure you implement the prototype in a manner which can be further reused in actual development.
3. Document the prototype
5. Use ready to use tools instead of using any high level language to create a tool to use in prototype. This will help to save time.
6. Create sample with sample data in it which can feed 2-3 reports.
7. Present the prototype to end users to get a feel of proposed solution.
8. Once done, Get sign off on prototype.

Remember prototype is a very small part of your actual development project. It needs to done fast. If Business Users like your prototype success of project is very near.