How to draw on a bitmap with a Windows Store application using C#

I cannot figure out how to draw on a bitmap in a Windows Store (RT) application.

I want to draw pixel by pixel, setting the color.

It looks like I need a WriteableBitmap, but I am unsure how to get the pixel format correct.

All the examples I have found seem to read a bitmap from file and extract the pixel format from there. I am not starting from file. I am starting from a blank bitmap.

Does anybody have a simple example of just image on a form, draw pixels on bitmap in the image?