Lots of assumptions and estimates! How accurate, I don't know. Basic process goes something like:

Estimate age of universe: 13.8B years.

Estimate expansion rate of universe, to get radius of OU. 46B light years.

Calculate area of sphere = 4π46^2.

Now estimate number of galaxies from observable size.

Now estimate the number of stars from number of galaxies.

Now multiple by number of atoms in our sun ...

Oh, and this assumes all hydrogen atoms.