The first step in the process is to use the SpeedTree Modeler to create the model you wish to use in Cinema4D. This can be as simple as opening a library model or as complex as creating a tree from scratch. The model should include material assignments, correct uv coordinates, and everything else you need to see the model exactly as it should appear in Cinema4D. The goal is to replicate the model exactly as it appears in the Modeler in Cinema4D. This includes features such as detail mapping, branch intersection blending, and wind.
Once the model is complete, select 'File→Export mesh…'. In the ensuing file selector, select 'Cinema 4D files' from the combo box labelled 'Save as type' and pick a filename for the export. The model will be processed and the following dialog will be presented. Select the options appropriate for your situation and press 'Ok' to write the '.c4d' file.
Because SpeedTree writes out Cinema4D files directly, the exported '.c4d' can be imported as you would any other .c4d file. Unlike 3ds Max and Maya, no further processing is necessary.