First, use the barrel of coal-tar with the fractionalizing still, just outside of the Chemist's house. Then, click the right side of the valve to the right twice, so the tar should be flowing at maximum. You should see the 'pressure' indicator on the left go up. When the indicator reaches the green region, click the right side of the valve on the left once to let out pressure. This should stall the indicator on the left. Now you can start to add more coal. If it's above, don't add coal until it drops. The bar on the bottom will start turning green. When it's full, you can stop adding coal and close the window on the top right to get your barrel of naphtha.
A very easy way to do this is:
- Turn the valve on the right all the way to the third tick (all the way to the right).
- Wait for the pressure gauge to land in the green zone.
- Turn the valve on the left to the second tick (in the middle) to halt the pressure in the green zone. If it landed in the red zone, you could turn the valve right to relieve some pressure, then back to the middle.
- The valves never need to be touched again, and the pressure gauge can be ignored.
- Add 2 coal, and then 1 additional coal whenever the heat gauge falls too low (below, or counterclockwise from, the green zone). Don't spam click the "add coal" button. If you add 4 at once, it is likely to overheat and you will lose your barrel of coal-tar. The process is fast and may use only 4-5 coal.
Unsurprisingly, a Barrel of naphtha is tied for the heaviest item in RuneScape, weighing at 32 kg, and letting a player hold 896 kg of weight, with a full inventory.