I would suggest that you actually put the foods on a separate plate and bring it to an altar with a picture or murti of Sri Krishna and offer it there. If you are not diksita, you may just chant the maha-mantra and with a prayerful spirit ask for the Lord to partake of the meal. Then take his remnants. This is the most beneficial method from the angle of bhakti. You may or may not chant prayers before partaking of the meal -- that is secondary to the importance of directly offering it first.

As is evident by now, there are plenty of diverging traditions. You would then have to qualify the "we" in "mantra which we should chant" -- who are the "we", in which tradition?