If you’re not quite to that level yet, that’s entirely OK. Just keep your feet on the floor, prop your arms up behind you, and focus on keeping your elbows as close as possible to you throughout the movement. You’ll start seeing improvements in no time.
When it comes to building serious biceps there's probably not a better movement than chin-ups. They’re a great movement for hitting nearly the entire body, but especially the biceps, thanks to doing the movement with your palms facing you.
But unassisted chin-ups aren’t for everyone, since they require a tremendous amount of upper body strength. Fortunately, there are plenty of other options.
One of my favorites is the TRX row with your palms facing upward. This allows you to do a scaled-down version of the chin-up, while forcing your biceps to do plenty of work. If you don’t have a TRX or similar set up, using any fixed object that you can pull against works perfectly fine.