the 20g suggestion is assuming distillation.
distill, then pour the distillate into a jar/beaker holding 20 g magnesium sulfate.
20g is a roundabout figure, should be able to handle a pint of vodka

he may need to add more magnesium sulfate, up to 100 g, depending on how much is being distilled
but yeah, it doesn't take much, after distilling

Some notes about distilling:
- Using a simple still without a column, triple distillation gets it close to 90%.
- Keep the very first and last part of each run apart (foreshots and feints), they can contain contaminants. Add this to the next batch to be distilled to increase alcohol yield.
- Distilling ethanol without a license is illegal in most countries. This is moonshine even if it's not for consumption!
- All about home distilling: http://homedistiller.org/
With some copper tubing and soldering it's not so hard to make an excellent reflux still that produces 96% ethanol in one go!