Place the lemon on its side on a cutting board.
Slice the lemon across its width into paper-thin rounds.
Pull out any seeds from the lemon slices with your fingers or the tip of the paring knife.
Press the lemon wheels around the base of the sides of the cake, using the cake's icing to hold the lemon slices to create a border on the sides of the cake. Arrange the slices on top of the cake in a spiral with each slice overlapping the one next to it to enhance the top of a round cake.
Hold the lemon with your nondominant hand and a vegetable peeler in the other.
Start the peeler at the tip of the lemon and slowly rotate the lemon while holding the peeler still to remove the lemon peel in a single strip. Use the leftover lemon in cooking, if desired.
Cut the lemon peel strip into shorter segments to make them more manageable on the cake. The size of the pieces of lemon peel will depend on whether you want long or short lemon curls on the cake.
Hold each lemon peel segment at either end and twist the ends in opposite directions to make a spiral.
Scatter the lemon peel curls around the sides and top of the cake for a casual look or arrange them around the edges of the cake with regular spacing between for a more formal cake.
Hold the lemon vertically on a cutting board.
Slice through the lemon lengthwise to cut it in half.
Place each half cut-side-down on the cutting board and cut each in half lengthwise. Cut each of the quarters lengthwise in half if you want thinner wedges.
Arrange the lemon wedges around the base of the cake, resting on the peel with the cut side facing up, or place the lemon wedges on their sides with one of the cut sides laying on the cake.