This being my first time I found that this step and the following steps to be the most difficult and time consuming. It required A LOT of patience because the rubber membrane and the adhesive holding the cable really don’t want to release without sufficient heating and I was concerned about overheating the assembly. Using a hair dryer on the slowest setting finally worked.
I found that the single suction cup from the kit worked very well. It didn’t take that much force to separate the front panel from the phone. I think any suction cup that you are able to get a firm grip on will probably work as well. There isn’t any adhesive holding the front panel in. It just snaps in and out. If you have a cracked screen and no screen protector, it may be hard to get a good suction seal. If that’s the case, someone else here suggested applying clear tape to the front glass to get a good surface for applying the suction cup. In my case, I already had a screen protector that was still intact so the suction cup sealed easily.