Pull down both sliders to the very bottom of the zipper and make sure that they are close together. Grip the fabric of the jacket, not the sliders. When you hold the fabric instead, the zipper with the sliders is kept straight and stable.
Grasp the opposite part of the zipper and insert it into the sliders until it stops at the bottom. Because you keep the jacket stable and straight in your other hand, this part will easily slide into the groove and position itself correctly.
Pull up the top slider to close the zipper.
The picture shows what it should look like when you put on your jacket.
The picture shows how not to hold the slider.