I don't actually fold them like most people do. If you bring the the opposite ends together and gently press them against each other, then slide them underneath the inside clips on the empty left hand side. Press the steelbook closed carefully, it doesn't crease the the middle of the J card. They remain unmarked and can be flattened with no marks.
If you look, there's no crease mark in the middle of the J card. It'll flatten perfectly well again, even after being in the steelbook for months and months.
Definitely works.