what are the halachos for using a blech on shabbos?

I am assuming you are referring to chazarah(returning food on shabbas) and not initial halachos Shehiyoh(leaving food on blech).Food that was put on the blech before shabbas and now you want to take some and return it to the blech you need 4 things to allow you to do it.1)food has to be fully cooked 2)you have to have in mind that you will return it to the blech when you take it off 3)the pot has to be held the entire time time when taken off until put back 4)has to be returned to a covered flame(blech).With regards to cooked food(non liquid) to warm up on shabbas some place near the blech or make a hefsek kedaria (pot barrier) to place kugel on top of a pot cover already on the blech and some even say one can place a clean pot or plate upside down(doesn’t look like cooking).A Rov MUST be consulted this is not psak halacha this is just to give a really basic overview of the topic since there are many cases and variables which must be known in order to know what to do.

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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.

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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.