Everything has to be done with seichel. The way I prepare my blech is as follows:
First the flames are turned as low as possible, one upper right and the other lower left sides. Then I put a perforated 1/2 in thick “simmer ring” over each burner, whether it is on or off. These rings are perforated metal discs that essentially distribute the heat of the burner so as to turn any pot over it into a double boiler. I put my blech over that, with tinfoil covering it, and the pots go on top of that, or over an upside down pan.The simmer rings allow the burned off gases to escape from under the blech. In 33 years B”H and bli ayin hara, I have never had a fire or carbon monoxide detector go off. Putting a metal blech directly over the burners with nothing intervening, might cause the blech to be so close to the fire that it can cause a buildup of carbon monoxide underneath. My blech and the rings underneath are raised, so there is good ventilation.