A big thanks to 53_robin for providing us with the solutions!
There are 6 stamps to collect. It is quite easy to complete it and you can earn 500np from completing it.
First stamp: Play the game three times to obtain it.
Second stamp: Complete the game once to obtain it.
Third stamp: Send your score twice to obtain it.
Fourth stamp: View the trailer twice to obtain it.
Fifth stamp: Visit the ‘About’ section to obtain it.
Sixth stamp: Visit the website twice to obtain it.