My iPhone would shut down when using any app that required power, even if the battery showed over 60% charge.
The step-by-step guide was extremely helpful. It took me about twice as long as what your tutorial projected, but I made a mistake and had to partially disassemble what I had already completed.
Pay close attention to the connections before you take them apart, especially connector #5. This one took a little extra time to figure out when reassembling. A close-up photo of this step in the tutorial would be helpful.