1- Login to your YouTube Channel
2- Select the Video Manager
3- Click Channel Settings from the sidebar and choose InVideo Programming from the drop down menu
4- Select the Add a Watermark options
5- Click Choose File button to upload the Subscribe button image (readily available from Google Images or make your own) and click Save
6- Choose when you want viewers to see the Subscribe button (e.g. At the end of Video, The beginning or at a custom of your choosing) and how long you want that button to be displayed for, then click Update
7- Viewers are now able to subscribe to your channel with one click.