Use an immersion blender to thoroughly blend the ground flax with the water.
Let sit for 5 minutes, it should become a bit gooey.
Notes
Even if you only need 1 flax egg, you may want to double the recipe so there’s enough mixture for your blender to mix.
The average volume of a typical hen’s egg is 3 tablespoons, or about 45 grams. This can be a good guideline to follow & measure out your flax egg if you’re using it in baking.