We say it in the first person in the present tense beginning with I am....... It's not about anything superficial rather something that we find within ourselves first then that intention radiates out to others.
It could be something that is kept very simple to begin.
It could be that you long for peace, peace for yourself and then peace for others. In this case, your sankalpa will be "I am peace".
If you feel like you're recovering from bad habits or ill health It could be that you want to feel nourished and healthy. In this case, when thinking about setting an intention it could be " I am nourished in every part of my being".
But it’s completely up to you what kind of sankalpa you want to do!