In this folder there are 2 examples: one for 2 buttons connected, the other for 5 buttons.
But really there's nothing hard: simply connect your pushbutton to GND and to a digital pin.
The schematics are made with the open source software KiCad so you can open them without any cost.
There is a pdf version of the schematics too, if you need only to look at them without modify them.