Pada-sevanam literally means to serve the lotus feet of the Lord. After sravanam (hearing), kirtanam (chanting) and smaranam (meditating), when one desires to express one’s devotion actively then it’s time for pada-sevanam. Pada-sevanam is active service to the Lord.
Pada-sevanam kind of overlaps with other processes of bhakti, especially with arcanam and dasyam. Pada-sevanam expresses itself in activities like going on pilgrimage, visiting holy places like Vrndavana, Jagannatha Puri and holy rivers like Ganges or Yamuna, cleaning the temple, offering food, offering something valuable, and serving the deity in general (arcanam).
It also includes serving the representative of the Lord, the spiritual master or the pure devotee, attending to his needs and assisting him as a menial servant.
If pilgrimage is not possible, temples or pure devotees are not available, and you haven’t installed an image of the Lord at home, then mentally performing menial service to the Lord is also an option.
You can massage His lotus feet, offer Him water, food and all kinds of amities in your mind with love and devotion. However, for a beginner it would better to do arcanam instead (the next item), which involves using your physical senses.