In preparing food, cleanliness is the most important principle. Nothing impure should be offered to God; so keep your kitchen very clean.
Always wash your hands thoroughly before entering the kitchen. While preparing food, do not taste it, for you are cooking the meal not for yourself but for the pleasure of Krishna. Arrange portions of the food on dinnerware kept especially for this purpose; no one but the Lord should eat from these dishes.
The easiest way to offer food is simply to pray, "My dear Lord Krishna, please accept this food," and to chant each of the following prayers three times while ringing a bell:
1. Prayer to Srila Prabhupada:
nama om vishnu-padaya krishna-preshthaya bhutale
srimate bhaktivedanta-swamin iti namine
namas te saraswate deve gaura-vani-pracharine
nirvishesha shunyavadi pashchatya desha tarine
"I offer my respectful obeisances unto His Divine Grace A. C. Bhaktivedanta Swami Prabhupada, who is very dear to Lord Krishna, having taken shelter at His lotus feet. Our respectful obeisances are unto you, O spiritual master, servant of Bhaktisiddhanta Sarasvati Gosvami. You are kindly preaching the message of Lord Caitanya deva and delivering the Western countries, which are filled with impersonalism and voidism."
2. Prayer to Lord Chaitanya:
namo maha-vadanyaya krishna-prema-pradaya te
krishnaya krishna-chaitanya-namne gaura-twishe namaha
"O most munificent incarnation! You are Krishna Himself appearing as Sri Krishna Caitanya Mahaprabhu. You have assumed the golden color of Srimati Radharani. and You are widely distributing pure love of Krishna. We offer our respectful obeisances unto You."
3. Prayer to Lord Krishna:
namo brahmanya-devaya go-brahmana-hitaya cha
jagad-dhitaya krishnaya govindaya namo namaha
"I offer my respectful obeisances unto Lord Krishna, who is the worshipable Deity for all brahmanas the well-wisher of the cows and the brahmanas and the benefactor of the whole world. I offer my repeated obeisances to the Personality of Godhead, known as Krishna and Govinda."
Remember that the real purpose of preparing and offering food to the Lord is to show your devotion and gratitude to Him. Krishna accepts your devotion, not the physical offering itself. God is complete in Himself--He doesn't need anything--but out of His immense kindness He allows us to offer food to Him so that we can develop our love for Him.
After offering the food to the Lord, wait at least five minutes for Him to partake of the preparations. Then you should transfer the food from the special dinnerware and wash the dishes and utensils you used for the offering. Now you and any guests may eat the prasadam. While you eat, try to appreciate the spiritual value of the food. Remember that because Krishna has accepted it, it is non-different from Him, and therefore by eating it you will become purified.
Everything you offer on your altar becomes prasadam, the mercy of the Lord. The flowers, the incense, the water, the food--everything you offer for the Lord's pleasure becomes spiritualized. When we offer the Lord something with genuine love and devotion, He enters into the offering, and thus the remnants are non-different from Him. So you should not only deeply respect the things you've offered, but you should distribute them to others as well.