The middle table may not end up having any extra fields. It may just end up being lookup fields from the Installations and the Installer(Employee) table. But if, for example you need to record the amount of time on the job, per installer, then you might need to add some fields.
A way to get the Many to many concept in your head is to think of a traditional order or invoice which has line items on it.
One Invoice has many invoice lines.
One Item Master has many invoice lines. (ie, obviously when you have an Item in your Item master, you hope to sell it more than once!)
The middle table sometimes called join table, would have lookup fields from the item master for the item code and the item description, but would typically have its own fields for the qty, and the it might have a lookup for the price which would be used in a snapshot field to freeze the price at order entry time and then a field for the extended price times qty. It might also have a Price override field.
So the middle table will have lookup fields, and then might also have it's own fields if they are needed.
