1-Back: Remember whether the position or color (or both) is the same as in the previous round, press the according button/buttons if they match.
2-Back: Remember whether position or color are the same as 2 rounds ago.
N-back: Remember whether position or color are the same as n rounds ago.
Z or <- for a position match
X or -> for a color match
P to pause (+ additional menu)
Try playing in standard mode to understand how the game works, in endless the game ends when you make a mistake or miss a match.
You can switch colors for letters in the additional menu (P) in case you're color blind or are more comfortable recognizing them.