Assuming you already did the basic guide, you can now go to assets/minecraft/textures/custom/item and add your textures.
Please note that textures must not exceed 256x256 pixels!
Step A: Using a base model
Step B: add your models
Next you will see where to add the file of your model, read the options to know what's next
Step A:
If you are using an item that does not use a custom model you can use one from Minecraft, First go to assets/minecraft/models/custom/item and add a file named <item_name>.json
If you are adding an item that uses a custom texture such as a 3D modeled stick, you will have to complete several steps, this guide assumes you are using Blockbench to create your models.
Blockbench makes exporting models very simple just click on File > Export > Export Block/Item Mode And then place it in the correct directory
Third step: Define the CustomModelData
Now that you have the model in your texture pack, you need to configure the CustomModelData of the model.
What is that, the CustomModelData is a type of variation that Minecraft has for all items so you can create new ones and not replace them.
To do that you need to create a model file for your item at assets/minecraft/models/item/<base_item>.json in this case it will be stick.
Last step: You're done!
Incredible, we have created our first custom item, now we can get it in the game using the command /give <name> stick{CustomModelData:1}