Remove the bolt from the stem and remove the stem cover as shown in the photos below.
Open the lever on the handlebars by pushing the button highlighted in the photo below.
Install the stem and handlebars as shown in the photos below. Using an Allen key fully tighten the handlebars as shown in photo 3 below. Now close the lever on the stem as shown in photo 4 below. The lever should be hard to close. If it closes very easily, it won’t fix the handlebars tightly. If the lever closes very easily or if it is too stiff to close, you can adjust its tightness, by tightening or loosening the bolt highlighted in photo 6, below
Tighten the 2 bolts on both sides of the stem as shown in the photo below. Please keep the handlebars perfectly perpendicular to the front wheel while tightening these bolts.