reserve a small portion of your old starter in your jar. this is called unfed starter.
discard the rest. i am a big proponent of zero waste, so i like to kill two birds with one stone by keeping the discard in an old yogurt container in the fridge to use for fun discard recipes like crackers or banana bread.
add water at to the jar. stir using a spoon, until starter seems evenly distributed in the water.
incorporate fresh flour. mix thoroughly with a spoon until flours are well incorporated.
repeat! i feed my starter once a day, but i know some folks feed theirs twice.