Just to the right of the Submit and Cancel buttons. If it's checked, then non-Admin users don't see the edit (including all the cards edited if it's a multi-edit). Better yet, implement add persistent user preferences and GUI for them and let people decide for themselves if they want to see minor edits.
Possibly, mark minor edits in change view (MediaWiki uses a bold m to distinguish them).
add minor edit checkbox when editing+discussed in support tickets
add minor edit checkbox when editing+relevant user stories