This is a good time to test that the circuits are all connected properly and that nothing is loose.
Connect a display, keyboard and mouse and power up the Pi; the LEDs on the SenseHAT should display a bright rainbow for a moment and then turn off in the usual way. You can test the system by opening a terminal window and typing
cd ~ wget https://github.com/raspberrypilearning/astro-pi-flight-case/raw/master/test_code/pygame_test.py --no-check-certificate chmod +x pygame_test.py ./pygame_test.py
If you wiggle the joystick and press the various buttons you should see the directions indicated and letters on the LEDs. Hit the ‘esc’ key to exit the program and then shutdown and disconnect the power, keyboard, mouse and display cables.
Finally, it’s time for final assembly