Bases, or Art bases, aka pixel dolls, are characters (human, anthro or animals) with little to no personal traits, created by artists and used by other artists as a base to personalise and alter. They're often a blank, unclothed and bald character with a specific pose, expression and sometimes with prompts and backgrounds. Usually those who uses bases are beginner artists who wishes to draw, but don't feel confident enough (or just don't want to) draw the base of the character. Sketching the proportions are then skipped, which makes the whole process much quicker and easier. Fandoms like MLP, Winx and anime had popularised these bases around the mid 2000's to the early 2010's.

There were a debate in the art community, focused on if using bases is cheating or not, if bases traced over cartoons/anime goes against copyrights, and if using bases will slow down artists' art progress. I personally don't care what your believes are on the matter, the only thing I'm heavily against of is turning someone's art into a base without their consent, including my art. Take the bases avialable here, use them however you want, just respect the few don'ts I've listed below, everything else is either allowed or I don't care if you do it.

Not allowed :

⇒ Making a base from my art works, only use bases avialable here.
⇒ Reposting the base without credits, regardless if it was altered or completed.
⇒ Selling the base
⇒ Using my base for commissions or YCH

If you want a custom made base, my commission are avialable.

There's no unique way to use bases, but here's a simple method. The only thing you'll need is an art program which has : layers, selection tools, bucket tool, pen and eraser.
Unless you can copy the base's art style, I advice you to trace over the whole base and not just sketch/line added elements. Using layers will help you not mix the base with your lineart, flats of colors and shading. It is possible to use bases on MS PAINT if the lines doesn't have anti-aliasing, but I advice you not. There are lots of free, easy to use programs such as Fire Alpaca on PCs, and Ibis Paint on phones and tablets.

I'll always two version of each bases : with and without anti aliasing.