A diaphragm covers the cervix, thus blocking the sperm from entering the uterus.
Because the blocking effect is not very reliable by itself, you must use spermicides before you insert it. You place some spermicidal cream or jelly in the dome of the diaphragm, spreading it around the rim.
You can insert it before lovemaking, but no longer than 6 hours before sex.
After intercourse you must leave it in place for 8 hours. Afterwards you wash it and dry it and use it again later.
It’s important that there is a proper fit. It requires a visit to a clinician trained in determining and prescribing the appropriate size. Never use the diaphragm of a friend: it won’t work.
After a delivery, an abortion or after gaining or losing weight (more than 6 pounds) you might need a different size. Go to a clinician to have it checked out.