Open the code menu
Click the Codes button in the bottom-left corner of the screen. A pop-up will appear with the code entry field.
Paste exact text
Codes are case-sensitive. Always copy/paste rather than typing — typos are the #1 reason a code fails.
Claim the high-value codes first
BETA100KVISITS (20,000 Cash), 10KMEMBERS (15,000 Cash), 200KMEMBERS and THANKSFORPLAYING (10,000 Cash each) are the biggest single redeems. Do those first.
If a code fails
Try logging out and back in, switching servers, or restarting the game. If it still fails, the code is probably expired. Report in the comments on codes sites so the community can flag it.