The strictest way of doing this fast is to take only the five products of the cow (pancha gavya) on each of the five days.
But usually those who do this fast restrict their eating like during ekadasis: no grains and beans.

Padma Purana says that on the fifth day of the Bhishma Panchaka in the night one can break the fast by taking fruits, roots
or 'havishya' (rice, mung dal, optionally certain varieties of saks, etc. Salt is okay but no spices).

-extracted from a text by Vidvan Gauranga dasa, 1991

This 5 day fast is also prescribed for the last 5 days of Kartika (Damodara) month. During that vrata puja offerings to the Lord are also prescribed (see below). There are also mantras that go with these offerings. However, I didn't find any specific mention about this for Purusottama month. In the Padma Purana it only stated that in addition to Ekadasi one can observe a "pancaratrika" fast until amavasya. Here are offerings prescribed for Bhisma Pancaka.

On the first day, one must offer padma (lotus) flowers to the feet of the Lord
On the second day, one must offer bilva (wood-apple) leaves of the thigh of the Lord
On the third day, one must offer gandha (scents) to the navel of the Lord
On the fourth day, one must offer java flower to the shoulders of the Lord
On the fifth day, one must offer malati flower to the head (siro-desa) of the Lord