There are many screws in the iPhone and each one varies in size. Their typical size is smaller than an eyeglass screw. These screws can easily be misplaced or put back in the wrong position causing issues later. There really aren’t many shortcuts with doing a repair of this nature. The utmost care and time must be taken from start to finish and even then damage can occur regularly.
A screen installation on the iPhone 4 and 4s models requires full disassembly to access the front panel. On top of the many screws that need to be removed, some of which in tricky spots, there is double sided tape holding the glass to the framing. You need to remove this without damaging any underlying components. Some of these components are actually attached to the glass that you are removing and are necessary for reinstallation on the new glass. We see many types of cable tears in various areas of the phone. The home button can tear, the top power button can tear, even when installing a new iPhone front glass if you are not careful you will tear the cables on the new glass LCD and digitizer.
The battery connector on the iPhone 4 and 4s models is rather delicate. We see breakage here quite regularly and even get calls from other shops having issues after breaking this particular connector. Any type of minor board damage can result in a completely useless logic board.
On older iPhones the connectors on the board can become spread apart and not make solid contact with peripheral connectors like the LCD and digitizer. Some of the cabling is intertwined very compactly in the corners of the casing. There aren’t as many screws as an iPhone 4s but 3G and 3Gs models do have a few nuances.
There are many more things to consider when fixing an iPhone so it is important to do all your research. Here is a video below that shows a basic repair on an iPhone 4 AT&T model. Keep in mind that these are just instructional videos and other models such as the Verizon CDMA version and iPhone 4S use different methods.