Grab a bar with one hand and with the other hand grab an elastic band anchored to the bar.
Perform a pullup, while aiming to use as little assistance as possible from the band.
Repeat on both sides.
The band provides assistance in a convenient and affordable way, but it is hard to track exactly how much assistance you are getting: how far you grab the band from the anchor point and how low you pull it, changes the assistance a lot. To reduce the uncertainty, try to be consistent in the way you use the band.
Form tips
Start with the working arm fully extended and finish with the arm fully bent and your head above the bar.
Control the descent the whole way; do not let yourself fall into the bottom position.