How did europeans import slaves to the Americas?

On ships, mostly from West Africa. The slaves were procured by all the local African chiefs. Slavery (although this is an uncomfortable truth for many of today's Africans) was rampant in Africa itself, and some 20-30% of all Africans at the time were traded and held as slaves by other Africans.

Of course this is no excuse at all for the Europeans' slave trading activities, but the happy partnerships with the local chiefs allowed them to simply place the order and sit back and wait for delivery.

Until the mid-eighteenth century, the English, Dutch and French were the major slave traders to the Americas. After around 1750, the English had pushed out all competition and became the main transporters, only allowing the Americans to take part of the trade.