Summarize this article:

The realities of childbirth are that you will probably do this during labor and may not even realize it, but if you don't, try to find a way to empty your bladder prior to massage. A full bladder may push the uterus off to the side, which makes the massage process both uncomfortable and ineffective. Uterine massages may take place immediately after birth, or may take places several minutes to several hours after delivery. Subsequently, it is a good idea to practice deep breathing and muscle relaxation immediately prior to and during the massage. Relax your muscle and take slow, calm breaths to help with the potential discomfort. If you still have a pain management drug such as your epidural in your system, you may not notice any discomfort. If your birthing professional opts to perform a uterine massage, trust that there is a good reason for it. It may cause some discomfort, but it is important that you allow your birthing team to perform the massage as they see necessary, since it could help keep you healthy and safe after birth. Just as you did through labor, you may choose to have a partner with you during the after-birth process, including the massage. Hold their hand or ask them to distract you to provide some comfort during the massage process.
Empty your bladder. Relax your body as much as possible. Allow the massage to happen. Ask for support.