While it is always important to follow the instructions provided on the tag, sometimes it is better to overrule the 'machine washable' certification and wash by hand. Please keep in mind, silk and certain vintage items should always be dry cleaned as it is (or may be) an incredibly delicate material.
First thing's first! Make sure your nails are trimmed and you have removed any jewelry that may snag the material. Fill a water basin with warm water and add approximately 1 tbsp. of gentle detergent. Add your delicates and gently knead them with your hands. You may allow them to soak for a few minutes if you would like as well. Once finished, rinse them in water with a splash of white vinegar (to help remove soap residue) and gently squeeze them to remove excess water. Never twist or wring out your intimates! Flatten and smooth on a hard surface to dry.
Machine washing delicates is almost a taboo, but certainly doable. The number one thing to remember is to always wash in a fine mesh laundry bag. For bras, be sure to clasp them so that they do not catch and snag your other item. Never wash in hot and be mindful of what you have in the same cycle, as some materials (like terrycloth) may be too harsh on your intimates. Use a gentle detergent and always wash using the delicate cycle option. Once washed, flatten and smooth on a hard surface to dry.