A mattress stain can be caused by a variety of things, and they are best removed when still wet, much like removing a stain from a carpet. However, you don't want to use too much of a solution to seep down into the actual padding of the mattress. Once the padding becomes wet, it will be difficult to completely dry. IOf it doesn't dry thoroughly, mildew or mold might grow. To prevent this situation from occurring, don't use the liquid when removing the stain, use only the suds.
Pour about 1/2-cup of laundry detergent into a large bowl.
Use a hand whisk to agitate the detergent as you slowly add warm water one tablespoon at a time. Whip until the bowl is filled with suds. You only want to create a lot of suds, so only use as much water as necessary.
Take the whipped suds and apply them to the stain on your mattress. Use the soft brush to scrub at the stain in a circular motion until it fades. Keep reapplying the suds as needed until the stain vanishes.
Allow the mattress to dry completely before making the bed.