In the video below, I show you how to add subscribe button at the bottom right of your YouTube videos.
To do this, go to the home page of your channel.
Then click on customize channel, then click on the branding tab.
At the bottom of the page, you can set a watermark for your videos. It should be 150 by 150 pixels in size. for the format, it must be png or gif.
Click on upload, and select the subscribe image from your PC, then click on Done.
If you want to use the same image as me, use this link, where you will find the subscription image. just left click on it and save it to your PC.
After uploading the subscribe button, you have three choices: you can either show the subscribe button at the end of the video, set a custom start time for it, or show it the whole video
For me, I set a custom start time after almost 1 min.
Finally click on publish.
Now if I view one of my videos, I can see that after 1min the subscribe button appear as expected.