Padi Puja is the event of worshiping the sacred 18 steps of Sabarimala at the abode of Lord ayyappa, Or those which are treated or considered to be those 18 steps , when performing puja for them one would refer to the same as Padipuja.
Below are a few Videos from youtube on Padipuja performed in Sabarimala which can give you a quick look. This may not be a complete puja but gives you an overall feel of how it is performed.