re: How to make the grooves on the top of a record cake

You could use a steel (or plastic) trowel like this one from Lowes: http://www.lowes.com/pd_114521-81-49867_0__?productId=3110329&Ntt=trowel&pl=1&currentURL=%3FNtt%3Dtrowel&facetInfo==

Or, a saw blade that is only used for cakes, of course. :) … or a serrated knife.

Then, center the cake on a turntable, set the trowel, and turn the cake. You COULD pipe the whole thing, and when finished piping, lay a piece of waxed paper over it, and lightly press down with a flat pan… Then, peel off the waxed paper.
For a STACK of records, you can do the same, and use the trowel around the edges.

Those are my suggestions. :) Hope your cake turns out great!