Let the elbow bend do the work
Choose a light dumbbell and stand comfortably, or use a suitable seated position if that is the version you have been taught. With the palm facing upwards or forwards as appropriate to the starting position, bend the elbow to bring the weight up, then lower it gradually. Keep the upper arm reasonably settled rather than swinging it forwards to finish the repetition.
The wrist should feel supported by the forearm, not folded backwards under the handle. If the handle is awkwardly thick or slippery, change the equipment rather than squeezing until the wrist position becomes uncomfortable. A smaller fixed dumbbell may be easier to manage than a bulky adjustable one at the same marked load.
Notice whether your torso rocks each time the weight starts moving. That often makes it difficult to tell how much of the repetition comes from the elbow movement you intended. Set the weight down and try a lighter option. The purpose of this practice is not to find every possible way to get the dumbbell higher.
Keep the return visible
Lower through a comfortable range instead of letting the weight drop once it has reached the top. You do not need to snap the elbow straight at the bottom. A brief settled position between repetitions can make the next one easier to judge, particularly if you have been building momentum without noticing it.
If you alternate arms, make sure the resting dumbbell remains controlled while you attend to the moving one. If you curl both together, allow enough space that the weights do not collide. Neither choice requires you to match another person's pace.
A curl that becomes a shoulder swing halfway through the set has changed character. End the set while the intended movement is still clear. Keeping the same hand position, standing arrangement and lowering control gives you a useful comparison next time, even when the dumbbell number stays exactly where it was.