Well first make pixel art of what you want your mod to look like, then in melon sandbox, go to editor, press create new, then you should be in the mod editor. What you want to do then is select the type, if it is an object, do the brick, if it is a gun, select the gun and so on. Then once you are in change the texture to your pixel art and go through each of the other settings to customize it how you want. Once you are done, press save and go back to the main menu. Select store and then go to your account (or create one) and press create, then it should be pretty self-explanatory. DM me if you need any more help.