My client bought this theme;

My client bought this theme;
http://themeforest.net/item/simon-wordpress-portfolio-theme/3236529

and hired me to build his website;
http://nickprowse.com

The Portfolio Tiles have titles (words appear when you mouse-over) in them, they are too big. When you open a tile, text is missing. You can't order the tiles the way you want. The ABOUT PAGE has missing text at bottom you have to scroll to see, dumb. The form fields on Contact page look different? There is no theme options to change this.

Before I started editing the CSS of the theme I would like some advice on best practice to do this without wrecking anything.

I plan to;
1) Back up SQL DB in cPanel
2) FTP back-up site

Then, what should I do?
Should I create a Child Theme first, and try to make changes to that. I have quite a few changes outside the normal settings of the Theme, just looking for advice on best strategy going forward to make this site look great quickly, thank you!