Self expression is a useful tool to have especially before going into labour. It allows you to collect some colostrum before your baby comes into this world, allowing you to start a stockpile of breastmilk for when you want to go out, or let your partner feed your baby. As an added bonus, if for any reason - doctors or nurses are worried about baby's blood sugar, or baby has jaundice, for example - you can give your baby expressed colostrum instead of formula.
Colostrum is what your breasts produce before they start making breastmilk. It is yellowish in colour, and has a very viscous and sticky consistency. Some liken it to honey. Colostrum is absolutely amazing and perfect for your baby, especially the first few days of breastfeeding, when your baby's stomach is the size of a grape, and their intestines have not been exposed to any bacteria. Colostrum is rich in antibodies, as your baby drinks it, it coats their stomach and gut - protecting your baby from bacteria. As a result, colostrum is high in protein, and lower in fat than in breastmilk. That being said, it is the perfect transitional food to the outside world because your baby has not had to use their digestive tracts in utero, so they get a few days to practice before breastmilk comes in.
When you start self expression, you may not collect anything the first few times. Don't be discouraged if you don't collect anything the first few attempts, or if you only collect drops. Collecting drops is amazing, and it will build up! You can collect the expressed milk into a cup or spoon, then use a syringe to collect all of it. Make sure you label it with the date before putting them into the freezer. Let your care providers know you have expressed milk to use instead of formula, so they can let you know when you need to use it.
Stanford Medicine made an instructional video that shows you the steps of how to hand expression. This is one way you can self express. There are other ways as well - including compressing the breast, pulling back towards the ribs, and pushing forward towards the nipples (instead of compressing more there), among others. Try different ways, some might work better than others, and which ever one works for you is the perfect one for you and your body.