Ok I see why he is giving the warning as he is incorporating a 120v system but he doesn‘t mention that at the beginning. But also he said he dropped a cable whilst connecting it. You are only doing 12 volt work and yes batteries do contain a lot of power that can cause you injuries but no more than many other things you deal with in daily life if precautions are not taken. Simply cover the other terminals so that none are exposed when making the connections. Remember Will and many others put their videos up so that we are all able to learn. They are simply passing on their skills and knowledge to enable us to do these projects.
You have taken a massive first step so believe in yourself because you are able to do it and when u have finished you will look back and wonder what you were worried about.
That aside, there may be aspects of the videos that provide you with some of the information that you need but you will need to filter out what is not applicable. Search through Will’s videos as he has some great ones on basic information to help you but it sounds like you are almost finished. Actually I’m wondering whether you have stumbled upon this site without seeing any.
https://www.youtube.com/channel/UCoj6RxIAQq8kmJme-5dnN0Q
This guy talkes fast but videos are packed with info search out all his others.
Good luck
By the way even though you have been advised to wear gloves when working on batteries, do not wear rings. Had a news report one time of a mechanic working on a car with a spanning in his hand. Whilst his hand provided protection to the spanner end from touching the other terminal the finger with his ring on that came into contact with the other terminal did not.
Tim