This is for taking 4:3 video clips and creating 16:9 projects. You do lose the top and bottom of your video, but there is no other way to get it to fit side to side without making everything short and fat.
Organize your 4:3 clips, but do not add them to your timeline yet.

Set the project properties to 16:9.
Place your clip on the Overlay Track.


Right click on the clip thumbnail

and select Original Size? This will make your clip short and wide. If this is video of your wife, keeping it in this format could end in divorce.

Right click again and select Keep Aspect Ratio? This will put everyone back into normal proportions. It will also automatically clip the top and bottom of your video.

When shooting concerts, this is great for cutting out the dead space above and below the stage. You can click and drag the video up and down to clip what you want, but be careful here. If you are using several clips, you could make the transitions between clips look strange when the center of your video changes from clip to clip.
Go to Share and create your video clip as you would normally, using 16:9 as the aspect ratio. Now you have converted that 4:3 clip into 16:9.
