My decorating instructor taught me this... years ago. Take an old towel and tear into strips... wide enough to fold in half. Put them in cold water and wring out. Pin them around the pan and bake as usual.
I used to do that... until I read about the flower nail method. I think it works better and it's much easier in my opinion.
i don't use strips around sheet pans. i use two flower nails. placed upside down, of course, one on either end of the pan. they work as a heating core, and my cakes come out nice and level every time. don't forget to gently press the top of the cake after it bakes, to push any air out or relieve any dome that may have formed. also, with your half sheets, knock the baking temp down about 25 degrees and bake just a bit longer.
HTH
It's not really that they don't come out of the cake, but certain cakes really stick to the nail. It is kind of hard to get the baked on cake out of the tiny crevices of the flower nail if it is not greased well. 
When you say place upside down. Do you mean you put them in the bottom of the tin then pour the cake mix in? Im not sure I understand how to use them to make the cake level?
When you say place upside down. Do you mean you put them in the bottom of the tin then pour the cake mix in? Im not sure I understand how to use them to make the cake level?
Yes, you place them upside down in the pan after you have greased/floured the pan and the nail. Use one or more nails depending on how big the pan is. Then pour the batter in and bake as usual. I don't quite know the science to how they work, but you won't get that big mound of cake in the middle. Use these instead of the heating core or cake strips.
HTH
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