Not all Arduinos can supply the 3.3v that the NRF24L01+ needs directly. For example, the Nano only supplies 3.3v if powered via USB.
To get 3.3v from a power source (such as what you would use to power the Arduino, or the 5v output of the Arduino), all you need is a regulator and a capacitor. For this, I used a LD1117v33 (archive.org) (the LM1117v33 (archive.org) is pretty much equivalent) together with a 10uF capacitor.
Comments / questions
There's currently no commenting functionality here. If you'd like to comment, please use Twitter and @me there. Thanks!