Stir together the warm water and flaxmeal. Let sit for a few minutes, then add it to the recipe just like you would do with any other egg.
Notes
Using finely-ground flaxseed matters! Some brands say that they are finely ground but are actually too bumpy to work. Finely ground flaxseed looks like a powder. If there are bigger lumps in your flaxseed, run it through the blender until it really is a powder. It is tempting to skip this step, but bumpy flaxseeds make bumpy baked goods that don't hold together, while finely ground flaxseed is an excellent egg replacer. So, if your flaxmeal isn't already finely ground, please include this important step!