You can add Subscribe Button on your Youtube Video very easily by following this steps.
1.Login to your Youtube channel.
2.Go to your channel and click on the video manager option.
3.Then choose the video from your videos on which you want to add subscribe button.
4.Click on the edit option on that video, a video editor page will be open.
5.Then click on the option annotations which above the video editor.
6.Then select note option from the right side option bar and write in description panel what you want to write on the subscribe button and modify the note to your required size, color and shape which is on your video.
7.Select the link check box then select the subscribe option which is below the description bar and type your youtube channel name in the text bar it automatically fetches your link and set the time where you want see the subscribe button.
That's it your now made your own subscribe button and you can save this button and can apply to other videos of your channel.