The majority of pieces are printed on one side only. The plain side is either folded inside (where it doesn’t show) or is ultimately a glue surface. The need for a two-sided element may occasionally arise, for example, a fence.
Design one face of the element completely, including tabs, folds, etc. Once the cut line is constructed, duplicate that cut line and flip it horizontally. Then complete the design of the reverse side, but never change the cut line unless it is to add a piece that will not affect the fit with the primary side. In the example above, a small flowerbed is added. The lower section will fold out to make a small glue surface where the fence mounts on the base. Remember that this reverse element will not need any tabs that duplicate the primary side. Doubling of tabs just adds bulk, and can cause an element to not fit as well. However, you may need to incorporate a 3D aspect (like the flowerbed above) in order for the piece to be firmly mounted on the model (i.e., no wobbly bits).