Well to me chants are like prayers only polished and requiring energy. I usually make some key points like if there's a deity I'm calling on, what it's for [protection] and who/what it's for [person, place, or thing] once I know that I jot down a few lines and rewrite until it sounds good. [Example 'earth, air, water, fire, bring me that which I desire' sounds nicer than 'air, fire, water, earth, help me get a car'] it doesn't have to rhyme but many feel rhyming gets you in the right mental state and helps you remember.
Prayers on the other hand, I usually start with 'Goddess within, Goddess without. God within, God without.' then say whatever I want to say [thanks for the food, please watch over my friend, thanks for the sunny day, so on] and finish with 'as in harm none, blessed be'. I find the opening and closing helps me start and end things [Like saying hello and goodbye] but the middle should be a heartfelt conversation, not a script which is what a chant should be.