"Wow a new awesome rom for my old phone!" you think. Then you find you have to flash it. Great. How are you supposed to do that? There are many recoveries that do this, like TWRP (Team Win Recovery Project). But then how do you flash the recovery? It's quite easy, actually.
Applications
1. Odin
Odin is the most popular way, but is only for Windows. Most of you would want this, as it is easiest. And if you want to run this in WINE for macOS or Linux, it is hard to do (believe me).
(Site looks scammy, but it is not)
2. Heimdall
Heimdall is basically an alternative for Odin, but the problem is most roms and recoveries are for Odin, not Heimdall.
Downloads:
Windows | IMPORTANT: Must have Microsoft Visual C++ 2012 redistributables installed!
Must install Heimdall before Heimdall Frontend and Frontend launched with heimdall-frontend; select packages for your Linux distro
3. JOdin
JOdin is really just Odin, but in Java and works on Windows, macOS, and Linux. The main difference between Odin and JOdin that you will notice is that it needs a PIT file. Just load everything in and click Start. It will ask you if you want to obtain the PIT file. Just click yes and start again.