Hi, Elephantus,
Maybe the following link will solve your problem. I don't know how to make floorplanning either, but I will try my best to understand how it works. Hope we both can make rapid progress in it. **broken link removed**
I just found an interesting link about Xilinx floorplanning. After carefully reviewing it, I recommend to our community. The author uses a 10-bit counter as an example and proposed 7 ways of floorplanning for comparisons. Although the Xilinx chip is out of date and the example is very simple, it showcased the basic concepts of carry logic, relative location, basic guidelines of FPGA floorplanning, and so on. It is a little lengthy, please read it with patience. I already learned a lot.
If you are floorplanning large devices, you may like Xilinx PlanAhead. It is software-assisted manual floorplanning. You can get an evaluation copy, or simply read the docs and watch the videos on their web site.