Level 3 and up The larger maps cannot be solved only by iteratively merging two rectangles. (Technically speaking, these maps are not sliceable.) You sometimes have to simultaneously merge groups of 5 or more maximal rectangles. (A maximal rectangle is a group of rectangles that together form a larger rectangle).
L-shape support Merging five or more (maximal) rectangles by hand is difficult - but of course you're welcom to try. Choose support level low to build the cartogram completely by yourself. Levels medium and high give you support for solving high degree merges: they allow you to build L-shapes, using up to 5 maximal rectangles for medium and up to 8 for high.
L-shapes All layouts in this game are L-shape destructible. That means that all high degree merges can be solved by iteratively adding rectangles to an L-shape. If two rectangles form an L-shape in the layout then matching a common edge and a vertex lets you group them into an L-shape. Once the rectangles are properly aligned you can click on the red cross on their common edge and the game will group them into an L-shape. You can undo this operation with a right-click into the L-shape. A number appears inside the L-shape that tells you how many maximal rectangles are grouped within it. You can use the mouse to change the shape of an L-shape.
Growing L-shapes You can add rectangles to an existing L-shape by matching a common edge and clicking on the red cross.