Xanthan gum should be used sparingly as a thickener – use too much and you will end up with an unpleasant gloop rather than a smooth flowing thick liquid. Xanthan dissolves better in warm liquids so if you wish to thicken a cold liquid, heat a little of it, add the xanthan gum to dissolve it then whisk in the remaining liquid. If you use too much xanthan gum in a recipe you may notice a heavy, gummy or even slimy texture in your baked goods- so measure carefully when using xanthan gum. Use a mechanical mixer for best results when mixing xanthan gum. This helps to maintain a constant speed that will prevent your recipe from getting chewy or clumpy.
Mix 1/8 tsp xanthan gum for each cup of liquid in your recipe. First, dissolve the xanthan gum in a little bit of water (or other liquid if your recipe does not call for water) and then mix it in with the larger batch. Do not try to mix it by hand, as it will “gum” almost instantly and form clumps. It must be in motion from the moment it is incorporated into the liquid.
From:
http://nouveauraw.com/special-raw-ingredients/xanthan-gum-thickener/