Quick answer: Dream Dust needs 1 green + 1 blue rune. The easiest pair is water (1 green) and blood (1 blue). Decompose the Science Kit from the lab chest at the Study Table, study water and blood, put a Firewood Shed in the lab and fuel it, then combine them at the lab. Take the dust back to the Plague Doctor and he gives you two free zombies.
This page covers dream dust and what do I need to research to get the dust made.
Open the alchemy tower
Use what the Plague Doctor gave you (Alchemy).
Get Science
Decompose the Science Kit from the chest at the Study Table.
Study water and blood
Water shows one green rune, blood one blue.
Fuel the lab and craft
The lab needs its own Firewood Shed (Fuel). "So, I can do it with just water and blood… one green thing, one blue thing."
Hand it to the Plague Doctor
He tests it and gives you two zombie workers (Zombies).
Sources: a recorded 100% playthrough and a 7-tips guide on YouTube, gk2help.