How to Make Ferrofluid in a Bottle
It is always easier to buy ferrofluid than to make it. Some of the steps don't work well unless the conditions are just right and it is hard to get them right on your first attempt. Luckily there are plenty of suppliers (this UK supplier is top of my Google search for "ferrofluid sales" and they also supply lots of cool magnets).

Ferrofluid is a suspension of small magnetic particles (often magnetite, the magnetic oxide of iron) in a fluid. To avoid the heavy and magnetic particles clumping together or precipitating, they need to be coated in a surfactant. And this only works if the particles are the right size (very small) and you have the right mix of surfactant that works in the carrier fluid (the fluid is usually mineral oil or vegetable oil; alcohols will be a poor choice).

Laserprinter toner isn't usually a mix of suspended magnetic particles: only specialty toner inks are (the ones used for machine readable printing). If you can get the right sort, it will save a lot of effort, but the wrong toner won't work at all and will just make a mess.

There are quite a few recipes on places such as Youtube, but some of them don't work well or don't explain some of the important details. So, if you don't want to spend a lot of time experimenting to get the synthesis right, just buy the ferrofluid.