When cleaning grass stains out of clothes it's best to use warm water because hot water can cause the grass stains to set in more on the clothes.
First soak the clothes with the grass stains in some warm water with some vinegar and the vinegar will help lift the grass stains off the clothes.
Then rub the clothes together to remove most of the grass stain and then put it in the washing machine and use the hot water setting on the clothes to help remove the rest of the grass stain.
You can also pour some rubbing alcohol onto the grass stain and let it sit and scrub the stain out or use some hydrogen peroxide on the grass stain and do the same as with scrubbing the grass stain.
Then put the clothes in the washing machine and let it run through a wash cycle.