Let's first see how to assemble the robot. The Mini Robot Rover Chassis comes as a kit, so for the basic assembly I will refer you to this excellent guide to assemble the main parts of the robot:

https://learn.adafruit.com/bluefruit-feather-robot/wiring-and-assembly

You can basically follow this guide till you obtain a result like on the following picture:

So far, you should have the motors and the wheels assembled on the robot, as well as the 3.7V LiPo battery & the FeatherWing doubler mounted on the robot. We are going to use the battery to power the ESP8266 WiFi chip and the Motor FeatherWing, and we'll use the FeatherWing Doubler to mount all the feather boards on the robot without using a lot of vertical space.

Now, we are going to take care about the motors of the robot. These will be powered by a larger power supply, that can drive the motors faster. First place four batteries (1.2V to 1.5V Alkaline or NimHA AA) into the 4 x AA battery holder, and then connect the battery holder to the motor FeatherWing component just as on the picture:

You can now place this FeatherWing on the FeatherWing doubler, as well as the ESP8266 Feather HUZZAH board. Also connect the two stepper motors to the motor FeatherWing. This should be the result at this stage:

Finally, mount the second stage of the robot using the metallic spacers, and also mount the battery holder on top of the robot:

Note that on the last picture, I already connected the 3.7V LiPo battery to the ESP8266 feather board. However, you can wait until the robot is fully configured to connect the battery. 

Congratulations, you just assembled your mobile robot based on the ESP8266 WiFi chip! In the next section, we are going to learn how to configure it so it can receive commands via WiFi.