The exact procedure for replacing a cracked Samsung phone's screen varies based on the model, but the process generally involves heating the screen's glue, prying the screen off and placing the new screen. Kits generally make this process simpler and come with instructions.
Most of Samsung's smartphones allow users to remove the batteries, SD cards and other components, making the phone easier to work with. Removing the screen involves melting the glue holding the screen in place. Heat guns generally accomplish this task quickly, but it's important to avoid overheating other parts of the phone. Hair dryers also work, but can take far longer.
Prying the screen off a phone takes time. Most kits come with plastic tools to gently pry at the screen, although many people use guitar picks as well. Having multiple plastic tools on hand can speed up this process by propping up pried sections of the screen and preventing the screen from adhering again.
After the screen is pried away, it's important to disconnect any buttons or other attached components. Repair kits often contain model-specific information about what to unplug. Kits also typically contain adhesive strips that make the process of installing the new screen simple, although it's also possible to use glue.