Eye protection, rubber gloves and an uncluttered workspace away from naked flames are important safety measures to avoid contamination and spillage. Many recipes for homemade windshield washer fluid contain methanol, rubbing alcohol or ammonia to stop the washer fluid freezing, and each of these are either mildly toxic or flammable.
Less toxic alternatives to methanol or ammonia are the addition of either vinegar or edible alcohol to homemade windshield washer fluid. Vinegar lowers the freezing point of water by 4 degrees Fahrenheit, and vodka or cake decorator alcohol have the same antifreeze effect as poisonous alcohol but without the harmful toxicity. Another good safety idea is to put one or two drops of bright food coloring into the homemade windshield washer fluid so it can't be mistaken for clean water and ingested.