Here is the help page for adding Captions for your YouTube video.
Log into YouTube.com
Click on the upper right hand corner on the profile icon, and then click on Creator Studio
Once inside of the Creator Studio, go into the Videos section, and select the video you wish to transcribe
Once you are taken to the video, click on the CC icon, which will take you to a new page.
Set the language of the video to the desired option
Click on English again
Choose Transcribe and set timings
Listen to the video and input the words as you hear them. Save as you go, and once you have completed this, hit Set Timings
Processing the timings make take a while. Once they are done, make sure to double check, and then submit.
Your video will now have captions.