Each chunkloader can load up to 81 chunks in a square. It has options for 1×1, 3×3, 5×5, 7×7, 9×9 chunks.
There are two different types:
To activate the chunkloader you will need a blaze rod.
Place down the iron or diamond block, anywhere is fine just be aware of the chunk borders so you are loading the expected chunks. (press F9 to see chunk outlines)
With a blaze rod in your hand right click the chunkloader you want to activate. Remember Diamond for always loading and Iron for online only.
Select the appropriate option for the number of chunks you want to load. After this step it will tell you a chunkloader was created. You're good to go!
Breaking the iron or diamond block will remove the chunkloader, you can also turn them off by clicking the redstone torch in the GUI.