dumbell row i find best doing near the end of a back workout, this way the weight i can use is lower anyway, so i can focus on using strict form, knowing that i already have done the raw semi-cheating heavy bb rows or t-bar rows first.
i'd say do 12-10 reps with them and you can do dumbell rows both for width and or thickness of the back, generally the more perpendicular to the floor you should row the dumbell to your hip (and this is mainly lat width) and the more uproght, you row the dumbell to the lower chest and focus on thickness. I prefer somewhere in between using a 45 dgree angle to equally tackle width and thickness .
Now if you consider yourself to be strong at dumbell rows, just take this contest, lower the weight with 10-20 percent and do the same sets/reps, just force yourselve to get the elbow as high and behind your back as possible while keeping the torso in a fixed postion. Now milk out this form until you got it perfect before you use your old weight again.
Important to try several benches and adjust their height and angle until you got a really stable platform,
also i stack several cushions on top of the bench to rest my arm on , this way i guarantee a favourable incline of the torso. A tip from a bro regarding my form i got, was to put your resting arm on the OPPOSING side of your torso, to counteract the tendency of the torso to use a twisiting motion to get the weight up