The Cuban press is an external rotation exercise to help build your rotator cuff strength up. This exercise can be done either standing or sitting. Heavy weight is not needed at all for this exercise to be effective.
Start by having your arms bent at 90 degrees, having your elbows at shoulder height, and your palms facing behind you. Lift the weight up by rotating your shoulders, so that your forearms become parallel to the floor and palms should be facing down. Keeping your arm in the same fixed position, continue to rotate your arms until your forearms are now vertical, hands in line with your ears, while still keeping a 90 degree bend in your arms. Keep your head up through the whole movement and take your time and do not rush each rep. This exercise is meant to build strength in smaller weaker muscles, weight is not important here.
If you are having a hard time understanding the form on the Cuban press try Youtube for an actual video.