Quick google search found some tutorials for you! Not sure if they are any good but hopefully they can show you how to actually do it:
Tutorials
Free Resources:
Now as someone who has done this before, I can give you the general basis of how to do it.
Step number one would be removing the default hotbar/backpack which can be done with this:
-- This is a local script inside of startergui
game:GetService('StarterGui'):SetCoreGuiEnabled(Enum.CoreGuiType.Backpack,false)
After that you would need to design the UI, whenever an item is added to the players backpack (Player.Backpack) you would clone a UI element you have created and put it on your custom hotbar.
When an item is added to the players character that means they have it equipped so you would change the UI item to whatever.
there are a lot of other things you’d have to do, such as when one of these UI items are activated, equip or unequip the item accordingly.
So yeah start by doing that and we’ll go from there.