A good latch means good attachment to your breast, not just your nipple.
It is a mouthful of breast that positions the nipple far back in your baby’s mouth. Their jaw is wide open and relaxed. When your baby nurses, their jaw moves from front to back, with a visible wave that starts at the chin and ends at the ear. You feel a rhythmic tugging on your nipple. Your baby’s cheeks, chin and nose have good contact with your breast.
A bad latch is when your baby is sucking on your nipple like a bottle nipple
The reason a bad latch doesn’t work is because breastmilk does not flow by mere suction, like with a bottle nipple. Instead milk flows with compressing and massaging your breast and nipple.
Similarly a breast pump doesn’t work by sucking out milk.
The intermittent vacuum stretches and releases the nipple, stimulating the release of oxytocin, contracting the smooth breast muscle, releasing the milk, and collecting it in a bottle. Yes, pumps suck, but they also release, and that is why they are different.
Practice. Practice. Practice.
The easiest way to get comfortable is to lean back against pillows, in a semi-reclining position with your baby in your arms. You can help your baby take the nipple with their mouth and nose very close to your nipple, or you can wait. A well-positioned and relaxed baby opens their mouth wide, and compresses your nipple deeply in their mouth.
Try not to latch and re-latch, over and over again.
Each time your baby clamps down, it bruises your nipple.
Your baby should be belly to belly. Check that their ear, shoulder and hip are in a line, body straight and square, and chin level, not tipped up or tucked down. Their nose, chin and both cheeks are touching your breast.
If it hurts, brace yourself. Wait for your baby to start suckling. When they are focused on nursing, you can make the adjustments you need to be comfortable, by gently shifting your baby's body, into a better position.