How do you read a palimpsest?

The same as any other parchment book, for that is what a palimpsest is (a recycled and rewritten parchment book). When scribes in the middle ages in Europe ran short on new blank parchment (writing leather) books to write on, they simply went to the library of the church, abbey, etc. where they worked and took an old parchment book at random that was believed worthless and scraped all the ink off its pages to make it blank again, this was called a palimpsest. They would then use this palimpsest to write their new document on. Many palimpsests got recycled this way dozens of times.

To read the old original text of a palimpsest was considered impossible (until recently) as that was the effect of scraping off the ink to reuse it. Also many palimpsests were rebound and pages folded and cut to fit the binding. So pieces of the original are often missing.