Like others here, I used a hair dryer on a low/medium setting. I kept my finger in the airflow as a temperature monitor and heated both the front and the rear of the assembly by regularly turning it around. I found that this just takes a lot of time. You have to allow the heat to travel through the plastic and glass into the glue. You then need to allow time for the glue to soften. Just be patient - it took me at least 20-30 minutes to get the two parts completely separated. If you have to protect your skin from burning you're getting everything FAR TOO HOT and you risk damaging (or even melting) some of the components of your lovely iPhone!
Just be prepared to take this VERY slowly. Allow the glue to soften, use your fingernail to initially find the join between the two components and gently, VERY GENTLY, keep the heat going and prise them apart. Once they're starting to part you can use a plastic spudger instead of your fingernail, but don't force it - be GENTLE. If you have any doubts, just continue to use your fingernail until it's started to separate all the way round the screen.
Be careful not to damage the rubber-like seal on the glass side of the assembly. Despite using my fingernail very carefully I inflicted (albeit minor) damage on a very small area of the seal. In the long term this might allow dust to find its way in between the glass and the display. If you can avoid damaging the seal you'll reduce the risk of dirt and dust ingress.
Initially I was clueless as to what part was supposed to be separated from what. It only became clear when I looked closely at the complete assembly edge-on. Even with my less-than-perfect eyes I could see that there was a join about half way through the edge. Heating it in the corner (as per the guide instructions) and gently pressing in a finger nail fully revealed the join. Once it starts to come apart, just work your way round the edge and you can eventually separate them enough to get a plastic spudger between the two parts to help prise them apart. NEVER force it. Just keep gently heating it (with your finger in place to tell you if things are getting too hot) and the glue will melt giving you a clean separation without risk of damage to either component.
An earlier poster suggested that a good way to start the separation of the two parts is to hold the rail and push on the glass on one of the long edges of the iPhone. I found this worked very well. If you have difficulty starting the separation in the corner (as per the guide), try this approach, but don't push too hard on the glass - especially if it is already broken.
I found step 11 a bit confusing. From the photo it really wasn't clear how the rails should be separated. And the instruction to "..insert a metal spudger between the two metal rails along the edge of the display assembly.." didn't really help.
I can't really describe what I did - once again, this is one of those cases where the photo should be clearer - but suffice to say that you need to closely examine the rail assembly, look at where the "joins" are between the rails and use the spudger accordingly. I found that gently levering the outer rail outwards away from the inner rail and slightly lifteng the display at the same time allowed me to separate the two rails and release the display assembly. Once it's starting to pop out of the rails you can slide it away from the ribbon cable end of the phone and remove it.
Once again I have successfully repaired an iPhone thanks to iFixit. My only criticism would be that the quality of the photos could be much better. The guide is written somewhat from the perspective of a person who has already completed the task several times. There's an implied assumption that those following the guide will somehow intuitively guess the detail behind the sometimes fairly basic instruction to do whatever is required.
Better quality photos (some taken at macro level) would greatly improve the clarity of the instructions. As things stand each of us doing this repair has to embark on a voyage of discovery and, to a certain extent, guesswork. I found myself thinking countless times "oh, that't what they mean...." Sure, the user's comments help to overcome this problem but, please, remember that a picture apparently paints a thousand words. Good quality close-up, higher resolution pictures would help a great deal.