92 Lesabre: how to replace the alternator?

I have done it as follows:
1, remove the cable from the battery
2, loose the belt from the alternator
3, take off the alternator from the blanket
4, remove the cable from the alternator
5, change a new alternator by reverse steps.

But, the alternator is not charging the battery when I started the car.
The battery and the alternator are new purchased. I also tried three time by changing the alternator three times. But the alternator is still not charging the batttery. Because the car turned off right away while I disconnect the negative terminal from the battery.

Anyone can advice me? Is there any mistake or missing step in the instaltion procedure I did?

Thanks a lot.