How to make fuel
Fuelmaking requires you to first learn the skill Fuelmaking.
Once you've learned that, you'll need to put together a fuelmaker machine.
To make a single fuel cell, you need 8 blocks of peat, 12 jellisacs, and 2 white gas (you get white gas by using a gassifier to transform general vapour into white gas; it takes 8 general vapour to make 1 white gas).
Once you have these ingredients, click on the fuelmaking machine and select "Use". It will pop up a menu with only one item; select that item and follow the directions.
Fuelmakers need fuel in order to run! Luckily, the machines start out 50 units of fuel upon assembly, and it only takes 1 unit of fuel to make 1 fuel cell. To refuel the machine, you'll need to be out of the operation menu. Click on the machine and select "refuel", or drag and drop a fuel cell from your inventory to the machine (drag and drop is faster).
You can make more than 1 cell at a time, and some upgrades will increase the number you can make at a time.
You also can occasionally get a fuel cell as a random drop from gardening. The higher your gardening skill, the more likely it is you'll get one.