Simply add a few drops of your favorite essential oil on the toilet paper roll and allow the scent to flow through the entire room each time someone takes a few sheets off.
Once you've decided on which essential you want to use, apply a small amount to the interior of the toilet paper tube using a dropper, as most of these oils are potent.
If you don't have a dropper, purchase an eyedropper, as you don't want to cover the toilet paper with essential oil and increase the risk of skin irritation.
Avoid diffusers or misted products, too. These can spray too much essential oil into the cardboard tube and increase the risk of too much getting onto the toilet paper.