During Upload

When you are uploading a video, there will be several tabs underneath the video preview that you can select. If you click on the one labeled Monetization, it will show you a page where you have the option to monetize the video.

To enable monetization on the video, you must change the Usage Policy to "Monetize in all countries" if it is not already selected.

Once "Monetize in all countries" is selected, you will then be able to select your ad preferences for that particular video:

We recommend checking all ad format options in order to maximize your CPM, but these settings are completely up to you.

Once you have configured the settings to your desires, click "Save" at the top and your video will now be monetized once the upload is complete.

After Upload

If you've already uploaded a video but want to check or change your monetization settings, you can do that by going to your Video Manager, and hitting the "Edit" button next to the thumbnail on the video you wish to review.

You can enable monetization and adjust ads under the Monetization tab here similar to the options that were available at upload:

The green  under the video preview indicates that the video is already monetized.  If that symbol is greyed out, you will need to select "Monetize in all countries" from the dropdown in order to make any changes to your ad settings at the bottom.

Checking Status

From your video manager page, you can easily see which of your uploads have been monetized by the green monetization icons on the right side on the page. 

A grey means the video has not yet been monetized.  A yellow icon indicates that monetization has been limited by YouTube as the content may not be advertiser-friendly.  You can click "Request Review" if you believe this is in error.  A icon lets you know that this video is not eligible to be monetized.