There's not an awful lot of combining required, but you need to do a few such things. You click on one of the items, such as Rooter's head, to make it show up larger on the left of the other inventory items. Then you drag the item you want to combine with that, such as a bolt, to that large representation. If you've done it right, it will show the combined item in the large view, and will also have a "separate" button so that you can undo it if you made a mistake.