Place the prepared nose bridge insert inside the mask, between the two layers of interface material.

Push the insert all the way into the seam, so it's as close to the top edge of the mask as possible.

Use the two marks you made in the previous step to align the insert and center it.

The insert should be slightly shorter than the distance between the two strap attachment points, and it should not extend beyond either point.

Use several pins to the hold the insert in place against the seam.