To build a device using Protogo in TechTool Pro 8, first choose TechTool Protogo from the Mode menu. This will open Protogo in a separate window. You can then choose your drive from the Devices column, and the type of device you want to build from the Profiles column. Then, click Build TechTool Protogo device to begin the setup process. (Note that the drive will be formatted first.) 

Once the drive has been built it can be set as the startup disk by holding the Option key at startup.