Place 1/4 cup of quinoa seeds in the grinder (adding any more will overload the grinder and prevent the seeds from being ground to a fine consistency). Using on/off pulses, process, shaking the grinder every few pulses to ensure an even grind, until the seeds are finely and evenly ground. Repeat with more seeds until you have the desired amount of flour.
1/4 cup whole quinoa seeds yields about 1/3 cup of fine quinoa flour. Store quinoa flour in an airtight container of the refrigerator for up to 6 months, or freeze for up to 1 year.
Note: I have tried grinding seeds in a blender and two types of food processors without good results. The coffee/spice grinder is the only device that will grind the seeds into a fine flour, The other appliances create a more coarse meal. Fortunately, the grinders are very affordable and can be used fro, well, spices and coffee, as well as grinding other whole grains into flour. A worthy purchase!
Here is a great one for under $15: Proctor Silex Coffee Grinder
Do I need to rinse my quinoa before grinding? (added May 14, 2012)
No. Virtually all quinoa that reaches consumers in North America and Europe has already had the saponin removed (this includes quinoa flour and quinoa flakes). The label will typically indicate if it has been pre-rinsed. When instructons are given to rinse quinoa these days, it is typically just a brief rinse to remove any trace amounts of saponin residue that may remain during processing before use.
If you are uncertain as to whether the quinoa you have purchased has had the saponin removed (for example, quinoa from a bulk foods container), reserve it for another use besides quinoa flour (and rinse the quinoa thoroughly when you are ready to use it in another preparation).