I don’t!! Blech’s scare me. I really don’t like them at all. The only time I use one is when Shabbos is Yom Tov and I want my stove top to cook on Yom Tov.
Ask your Local Orthodox Rabbi.
But as far as I remember/do – Ain Bishul achar Afiyah. (there is no cooking after baking) So putting dry foods such as challah on the blech to warm is fine – but I usually put it next to or half off the blech away from the actual flame. The upside down plate/pan thing I never did, but is fine. There is also the issue of Yad soledes bo, the food can’t be able to get hot enough to hurt your hand.