Retyping by hand! The Formulas can directly be checked, because it's a computational language. Therefore there are no typos in the formulas possible:) So it's a lot of work, but some articles are jewels. I did a lot on Euler's work and there are a lot of typos in the formulas of Euler's original papers written in Latin.
The process can be accelerated if more than one person works on it parallel:)
