Once logged in to your MV account, click on the circle profile icon at the top right of the page and select: My MV> Upload Vids
You would want to select the Custom vid you want to upload and click 'Start Upload'.
The vid will go through the upload process, and when ready to be edited the option 'Last Step' will be available for you to click.
From there, you will have the ability to add a Title, Description, Category tags etc.
Don't forget to click the 'Custom Vid' option and the username of the member you want the custom vid delivered to.