Make sure that you are comfortable first, whether you are sitting down or lying on your side. Your baby should be held close to you and across your chest. His head must face your breast, with his chin touching your breast and his nose or top lip making contact with your nipple. Your baby's head and shoulders should be in alignment. Your baby should have no difficulty accessing your nipple, without having to stretch forward or turn his neck.
It is better to move your baby towards your breast, rather than your breast towards your baby – this increases the chance of good suction. If you find that it helps, you can hold your breast underneath with one hand. Use whichever position you are most comfortable with, whether it is cradling your baby, lying on your side, or holding your baby under your arm.