The single most important factor influencing whether or not the baby eventually latches on is a good milk supply. If the mother’s supply is abundant, the baby will latch on by 4 to 8 weeks of life no matter what in almost all cases.
Start on the breast she seems to prefer, or the breast that has more milk, or the side you feel most comfortable with if neither of the previous apply. If the baby latches on, she will start sucking and drinking. If she sucks only once or twice at the breast, she hasn’t latched on at all.
Do not try to force her to stay on the breast, it won’t work; move her away from the breast, calm her down and start again. It is better to go on-off several times than to force her onto the breast.
If the baby doesn’t latch on, finish the feeding with whatever method you find easiest. Cup and finger feeding works well and is better than a bottle.