Is there a thread, post, or wiki that explains how to find cheats? I can find basic stuff like inf hp, but I don't know how people make "all weapons" or "all items 99" codes in just a few lines. Also, are the hit addresses the same even after turning off the 3ds?
HP / Lives and items are no different. You're still looking for a number. If you have "10 health kits", you search for 10, get hurt, use a health kit, search for 9, repeat until you find the relevant address.
Then you need to determine whether that "health kit" has it's own item value (eg 01 might be a small health kit, 02 might be a large health kit). Once you have that figured out, you can write a simple code that writes the values to memory locations.
Have a look at the all items code found here:
http://fort42.cu.cc/
Study how they work and you'll figure it out.
Here is a simple 99 all items code I wrote:
http://fort42.cu.cc/gateshark/game1110/903/ It quite literally writes 0x63 to each memory location one after the other.
Here is a slightly more complex code I wrote:
http://fort42.cu.cc/gateshark/game1110/902/ The details are provided in the link to explain the code.
Edit: A lot of games use static addressing - so the memory locations always remain the same.
There are some games that use dynamic addressing (ie Pointers), and these will change each time you run the game. When making cheats,
always test your cheats several times by quitting and restarting the game, to ensure the memory locations are static.