Do you have a scale, istara? Get a cup that you think might be about right, and get some bottled water. Place the cup on the scale, level it, and pour water in until you get about 250 grams of water. Since pure water weighs about 1 gram per millilitre (0.997g/mL at 25 degrees C), you now have a measured out 250mL (of course, for an exact cup, weigh out to about 236.5 grams). This is not completely exact, but close enough as to make no difference in cooking. Grab a permanent marker and mark out a line at the water level, and there you go!
If you don’t have access to a scale…well then, I dunno. 