Ok andreas. We all know you refuse to read tutorials, so to sum everything up:
A good place to start is the mini tower item for your completed combinations that you will use to create the structures.
For the actual item stacking, you probably want two items, one that the player picks up (Probably a Powerup) and one that will stay in your inventory. When the Unit picks Up Item A, you give the unit Item B, or add a charge to Item B.
For Example, Both Item A and Item B are Sticks. The 'Item A' Stick is what you pick up and then you add charges to your 'Item B' Sticks, contained in the inventory.
To combine items, create an ability then make a trigger that checks for charges and items type and then remove charges/items based on your requirements then give the hero the structure item.