Ok, there are two major important links you need.
First is the smashboards thread about texture hacking: Here
Second is a Youtube tutorial: Here
In the youtube video description, there's a link for downloading the codes onto your sd card. When you have downloaded the codes onto your sd card, you go into your sd card and find the ocarina file. You start the ocarina code manager, and store the hacks to your sd card. Now you have the codes for hacking.
Next is to get the actual texture hacks. You need these websites to get texture hacks(though there are plenty others, these are just the most commonly used ones.)
Stack smash
Kitty Corp
You download the texture files that you want into specific folders of your sd card. The smashboards guide has a very detailed explanation on it.
After all that, you just load up the Gecko OS through homebrew, and it should work. The youtube and smashboards guide provide more than enough of a tutorial. If you need any more help or have questions, I'll be happy to answer your call.