a TNTOR - 2500 mAh power bank or similar 5V power bank that has a small form function - 17 euro. This one lasts at least a couple of hours running the pi in the way I was.
a micro sd card reader for your computer so you can configure the wifi password on the SD card for setup (or connect your pi to a display + keyboard directly)
Step 1: Attach the GPIO header to the raspberry pi