It won't be based off made up stuff, it will be based of the appendices of LotR.
And:In the book, there was no elves at Helm's Deep (except Legolas).
Faramir let Frodo, Sam and Gollum go along their way. He never took them to Osgiliath. Faramir was afraid of The Ring in the book.
In the movie, there was a scene where citizens of Edoras were evacuating to Helm's Deep, got attacked by wolves and Aragorn fell off a cliff. That wasn't in the book.
Rohan was winning the battle of the Hornburg before Gandalf arrived in the book.
The Ents decided to attack Isengard immediately in the book, instead of do nothing.
Wormtounge dropped the Palantir out of a window of Orthanc in the book. In the movie, Wormtongue stabbed Saruman in the back at the top of Orthanc (this is in the third movie, extended edition and happens outside Bag End in the book), Legolas shoots Wormtongue with an arrow and Saruman falls from the top of the tower while holding the Palantir.
In the movie, Theoden led the citizens of Edoras to Helm's Deep as a refuge. In the book, he was leading an army toward Isengard. He went to Helm's Deep because men of an army being led by Erkenbrand were attacked by a large army from Isengard and fled to Helm's Deep. Isengard's army was following Erkenbrand's scattered army to Helm's Deep, so Theoden led his army to Helm's Deep to help Erkenbrand.
In the movie, the Ents broke a dam and flooded Isengard. In the b ook, they dug trenches leading into Isengard, converged many rivers together, and made a dam to hold the water back. After the water builded up, the dam was broken and water poured into Isengard
And theres more. But I think I've said enough.