The diaphragm is a flexible, cup-shaped device that is made of rubber. The diaphragm is inserted into the vagina and placed over the cervix before intercourse. It is often used with jellies or creams to increase birth control effectiveness. The diaphragm, then, acts like a barrier to the sperm to prevent them from entering the uterus. Obviously the diaphragm must be placed correctly over the cervix.
Proper placement requires practice and also requires a diaphragm that fits properly.
Diaphragms have to be prescribed and fit by a doctor. You can't borrow a friend's diaphragm. Also, pregnancy might change the size of your cervix, so you will need a new diaphragm if you have had a baby. The effectiveness will decrease significantly if the diaphragm is not fitted or placed properly. There are some chemical methods of birth control that involve using foam, jellies or creams in the vagina to destroy sperm.