Enchanted books (normal ones, like Sharpness IV) can be placed on unenchanted tools/weapons/armor to enchant. Placing them on something already enchanted won't apply it. Placing a Sharpness I book on a Sharpness I sword won't make the sword Sharpness II.
Enchanting things in an enchanting table will randomly give you one or more enchantments.
To combine enchantments, you have to use the Jartex Magician, which costs anywhere from 500-6000 exp depending on the number of enchantments being combined. To upgrade an enchantment's level, you also have to use the Jartex Magician - combine two enchanted items of the same level (books don't work in /magician) and pay an exp fee ranging from 500-6000 exp depending on the number of enchantments being transferred.
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There is no real way to get all the enchantments you want on an item without spending a lot of exp.
That said, you could try killing villagers for emeralds, then trading those emeralds with a Librarian villager for a book you want. You can use basic enchantment books from /enchanter in the trade menu, or you can get books by buying bookcases in the misc. section of /shop. Then, once you have the books you want, get multiple copies of the tool/armor/weapon you want to enchant and apply each enchantment to each copy. Finally, you can then /magician everything together to get a fully enchanted item.
The above method should not waste too much exp, since you'll get exp by killing villagers for emeralds. It does take longer than enchanting in an enchanting table, but it's less reliant on luck.