There is confusion on how this achievement is gained among players when looking at the achievement picture. There are two ways to light an unlit torch or lantern in a level, the first way is to use your tnt and the other way is with flares.
Thanks to an update, tnt can ignite foliage in its blast radius, if any lit grass crosses paths with an unlit torch/lantern it will light them. This is admittedly rare and is less than recommended.
Flares can only be used in the Night Run mode
*** Spoiler - click to reveal ***The Night mode is the first game mode you will unlock.
You can unlock it by reaching level 20 in a classic mode run. (This can be the classic run, the classic seeded run but also the daily run)
In Night mode, you must traverse a level in darkness with nothing to aid you than a flare and tnt. Flares can be carried in your hand and cast a bright and wide light. You can also carry it on your back which will cast a small radius of light and will be dimmer than normal.
You can only light torches when carrying a flare in your hand while passing over the unlit item in question. Do this 100 times for the chievo over any number of runs.
For a full guide on Night mode, please refer to the guide written by the developer and my friend, Retrific
https://steamcommunity.com/sharedfiles/filedetails/?id=44842...