I know there was a video on this but I couldn't quite find it.

I went in to try a Gretsch yesterday at a local music store and when I was explaining that I was having difficulty with fretting the A and B chords with my fat fingers he grabbed my thumb and moved it down. I've always had my thumb either pointing towards the head of the guitar or if straight up my hand has been pulled back so that my curve on the strings isn't quite straight down. I wanted to watch more on the exact positioning close up. Just want to correct any bad habits before I play for years like this. And do some people find different ways that work for them better because I find it hard to keep my hand in the position he showed me before it gets fatigued(I also am not used to it so my hand isn't quite as accurate as it was)

Also I'm trying to learn muting techniques. Been playing the riff to Daytripper and I get a lot of sympathetic vibration as well as strings that I'm probably not muting after playing them. Do you actually touch the strings with your strumming hand after each note to stop it from continuing to ring or is it the way you pull your finger off the string that mutes it with with fretting hand?