Do what you can and keep your back flat. I did these last night using dumbbells for the first time, which felt easier to me but also felt like I was getting a better workout than when I used a barbell. My hamstrings are definitely sore today.
I don't do 'straight leg deadlifts' with an actual straight leg, since I have a tendency to lock up at the knees - not good. Therefore, I keep a slight bend in the knee while doing this - I also use a step with a couple of risers so I get a better range of motion.
Here's a link to Krista's website that shows both bad and good form on deads: