I want to start off by saying that this is what I do and I have never had a problem. All this was made by me and I took the time to make it so please show some support it would be very Nice!

Step One: Change the Email.

You want to do this first because if you change the password, or information like that it is going to send an email to there account saying the password has changed and they can click a link saying they haven't changes the pass and get it back.
Settings > Email > Change > Edit

Step Two: Change the Pass.

Of course you want to change the pass as soon as possible. If not they can easily get the account back. Make sure its not something easy, cause its possible you can get brute forced, or lucky guessed.
Settings > Change Password

Step Three: SIGN ALL SESSIONS OUT

I have delt with people who bought channels from me who did not do this crucial step. If you don't sign out all of the sessions, the person can still be logged on the account even though you changed the pass and email. MAKE SURE YOU DO THIS STEP! They can close the channel on you!
Settings > Advanced > SIGN OUT ALL YOUTUBE SESSIONS!

Step Four: Make sure you add on/ take off Security Questions.

The person can easily get the channel back if you don't change this all they need to do is say forgot password then answer the question. Make sure you take theirs off and add something on there that only you know not your mothers maiden name because people can and will find it out!
Settings > Email > Change > Security > Account Recovery Options

Step Five: Add on / take off phone number.

This is another easy way they can get the channel back. Make sure you take there number off of the account and put yours on. If you don't feel safe putting on your own phone number, download the app TextNow for iPhone and possible Android It gives you a phone number than allows you to receive texts and calls. Make sure your active on the number or your number will be dropped and forced to lose the number!
Settings > Email > Change > Security > 2-Step Verification.

Step Six: Reset Backup Codes.

I have never had a problem with this, but this is a way to login the account if you have the password and don't have the phone number on the account you can use one of the ten backup codes. Just to play it safe, Generate new codes just in case you have a sneaky bastard on your hands ;D.
Settings > Email > Change > Security > 2-Step Verification > Show backup codes > Generate new Backup Codes.

Step Seven: Add an alternate email.

Just to play it completely safe add on an alternate email. It doesn't hurt to add on something its better than regretting not adding it on. Its simply yet could be very helpful in the future.
Settings > Email > Change > Edit

Guys This is my knowledge of securing youtube channels, most of it might seem like common sense, but not everyone is as smart as you people who are probably going to flame me for making this post. I'm new to the community, and I wanna make a name for myself, so I figure tutorials like this will help you guys gain more respect towards me. This post took me over a half hour so some support/ appreciation is more than accepted . If I missed anything let me know and I will add on it.