I've found that i've been recieving alot of messages regarding how to add cheats by beginner codders. So I thought I should create this thread to help the beginners. It also explains how to do an Actor Replacement in short.
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Adding Gameshark Codes into Project 64 can be easy once you catch the hang of things.

First we want to make sure we have a code. So lets make one. For example we will find out how to make the rock humper turn into a arwing using the Flash Actor Hacker.

First we open up the actor hacker here: http://gekinzuku.com/GShacking/FlashGen.php

Now, we want to choose our location. For my example I am going to choose Kokiri forest because thats where my actor is located. Then I will press the button that says "ACTORS." We want to choose "Rock Humper" under the  "Select an Actor to Modify" list. Then choose your replacement (Arwing) in the list below it. Click Modify beside the "Select an actor to modify" list and then the other Modify button beside no-cutscene.

After this we can press "Create Code". Simply copy and paste the new code and make sure it looks like the following:

81244124 013B
81244132 0000

Now we open up Project 64. Simply go to Options > Gameshark codes (Cheats). A window should open up and there should be a tab beside Cheat List that says "New" click it. Place your name of the code and in the big box paste your code. Add a description and press Okay or Save, whichever it is. Then go back to your cheat list and tick your newly created cheat. Then play your game.

NOTE: You may need to go into Options > Settings > General or Options and untick the Hide Advanced Options. This is so you can input cheats.

Hope this helps,

Kayos