It is assumed that you have installed the servos with the center trim corresponding to center flaperon TRAVEL. This means that your flaperons will point down when the servos are in their neutral position, and that the lower flap position (full brakes) will correspond to one endpoint of the servo's, and the upper aileron position will correspond to the other endpoint of the servos.
The easiest way to achieve this is to use a servo tester with center, min and max outputs during build, but you can also use the transmitter if you know how to set it up for that purpose.
We will use the modelSim (thanks Miami Mike on RCgroups) to show and illustrate the process, and we will use the helpfiles and menu's to walk you through step by step.
So let's Go!