One should install the Deity of Lord Visnu or Krsna and worship Him nicely by dressing Him, decorating Him with flower garlands, and offering Him all kinds of fruits, flowers and cooked food, nicely prepared with ghee, sugar and grains. One should also offer a flame, incense and so on, while ringing a bell, as prescribed. This is called worship of the Lord. Here it is recommended that one observe the vow of subsisting only by drinking milk. This is called payo-vrata. As we generally perform devotional service on Ekadasi by not eating grains, it is generally recommended that on Dvadasi one not consume anything but milk. payo-vrata and arcana devotional service to the Supreme Lord should be performed with a pure devotional attitude (bhaktya). Without bhakti, one cannot worship the Supreme Personality of Godhead. Bhaktya mam abhijanati yavan yas casmi tattvatah [Bg. 18.55]. If one wants to know the Supreme Personality of Godhead and be directly connected with Him, knowing what He wants to eat and how He is satisfied, one must take to the process of bhakti. As recommended here also, bhaktya paramayanvitah: one should be surcharged with unalloyed devotional service.