Creating Child themes

Hi,   I noticed that when I updated my theme I lost all the additional CSS to the theme. which is really annoying!   And I read about creating Child Themes to avoid this issue. Can you advise? Is there I way I can find again all the additional CSS I had before I updated, or is it lost and I have to re-build? And is there a link you can show me where I can make a child theme so that this doesn't happen again? Thanks