...I use a piece of steel wire + friction to burn in a line on the outside of a bowl. To burn the edge of a plate, bowl, etc. I use the back side of crocus cloth. To burn a a circle on a flat surface I double over the steel wire as to form a smooth bend and crank the lathe up to 3,000 rpm. It is suggested that you hold said wire w/pliers.

There's always the "cheater burn." Sharpen your carpenter's pencil and hold it against the spinning wood.

Then there's the "hot pipe" method: heat up a metal pipe and "brand" the piece. This usually requires either a torch or a stove/fire to heat up the pipe.

HI guys
I like the look of burned lines but I prefer mine to be much thiner. I use guitar strings attached to a small piece of wood on each end. Using guitar strings allowes you to have one or many lines on your piece of work. You can also have the wires as long or as short as you want.

... Lowe's, Home Depot, or for that matter any home improvement and/or building supply store should have the formica samples that you can use to burn on the bottoms of you pieces.