What do you need to use a looper pedal?
Loop pedals record audio that is fed into them using either an XLR cable or a 1/4" instrument cable. They usually capture this audio as a WAV file, either using on-board memory or via an external SD card. They can then instantly play back this recorded audio when a musician steps on one of the pedal's switches.
Where does a looper pedal go in the chain?
Loopers aren't effects—they're recorders. Normally, you'll want the looper to be able to record and playback any of your sounds. Of course, this means you should place it at the very end of the chain so it can hear and record whatever pedal combinations you use.