I have made a video about how to do the replacement step by step.
in order to do a professional work, you should have
gorilla glass replacement + heat gun + infrared thermometer + cards + uv glue + uv light
+and few screen drivers etc, the rest I explain those things in these videos.
I hope it helps you so you wont mess up the screen of your cellphone.
here I post the videos (both are 30 minutes each and they have very well detail):

samsung galaxy s4 changing broken glass with uv glue


Theory of how to fix gorilla glass at any samsung galaxy


best regards
Gabriel

PD:
if you want something quick to fix just change the whole screen with lcd but
it will cost you more but it will save you a lot of time



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