how do you make a 12 volt heating element?

may be you can convert 220 v element to 12 volt element but I (Amp) increse.

long strip or wire of bad conductor (like steel packaging band used to pack bricks / thin iron wire / nicrome wire)

to convert 120 / 240 V heating element add 8 / 18 center taps and connect + - to them in alterating order. less taps for lower power

12 V lightbulb in a box

high power 12 / 24 V halogen floodlight - not exactly heating element but heats alot everything placed in front of it

heating elements are usually high power. at 12 V they will take huge current (A = W / V)