How to change a theme into a child theme

Hi Everyone. New to WordPress. I just downloaded the Buttercream theme off of here. I was going to make some major alterations but I read that its better to make a child theme so when you update WordPress you do not loose those changes.

Anyways, Before I make any alterations to the theme Buttercream…I’d like to change it to a child theme.

Can someone direct me to a detailed guide on how to do so. I am confused on how to change it to Child Theme and then what do you upload and what becomes Parent Theme. Super confused to say the least… any help is much appreciated. Thanks in advance!