doing chest fly is easy if the set-up is appropriate. if you are going to perform higher reps(around more than 15 reps) you can use a bench but, if you are going low reps(around 15 to 6), it is a must to perform it on the ground to prevent any chest injuries. when you do fly with dumbells on the ground, you can focus on your chest greatly since you do not need to waste any considerable energy for stability. you can also do reps to failure without needing someone to spot you but, these things do not mean you should perform it in any form and condition. make sure the degree between your arm and torso(abduction/adduction degree) is between around 80 to 45, also make sure your elbow is not bent too much or too straight(between around 170 to 150 degrees is good). you are going to feel it in your biceps a little but that is normal, also make sure you do not change the degree of your elbow and keep it the same during the whole range of movement to deactivate triceps. do not bend the neck and wrist to any direction and keep them neutral. to keep the chest muscle(pectoralis major) under constant tension, do not come up all the way and only come up as much as you still feel a good tension in your chest(the more you bring your arms perpendicular to the ground, the less tension on your muscle). also for the safety of your shoulder, do not rotate the dumbbells on top by bringing your thumbs close to each other(excessive pronation which causes excessive internal rotation of the shoulder). as a cue about how heavy you should lift, make sure the weight is bearable with your biceps. majority of people have a stronger chest compared to biceps so to be able to keep a good form and more importantly prevent injuries, make sure the weight feels right on your biceps.