[Samsung Galaxy S (4G Model)] jeffsf:
...you need to make sure you can make it through this checklist.

This isn't a "how to" -- there are plenty of threads about that and stickies and search work pretty well, as does the SGS4G Wiki.

1) How long have you owned your phone? If it is brand-spanking new and your carrier will exchange it if it turns out to be defective, stop here.

2) Why are you contemplating installing whatever it is? Are you confident that the developer knows what they are doing? There is a wide range of skill and an even wider range of testing of things you might find on XDA.

2) Do you understand that virtually all ROMs, tweaks, kernels, what have you that you find on XDA, are all experimental, without warranty, and may turn your phone permanently into an expensive paperweight? Yes you can literally fry the innards of a phone with bad software, or even inappropriate user settings.

3) Are you prepared to be without your phone being functional for hours, days, or longer? Expect it. You're thinking of flashing experimental software.

4) Is your computer stable and reliable? Do you have or have you ever had any issues with USB or Internet connectivity? Fix those first.

5) Have you already installed and know how to use adb? Have you confirmed that it works with your computer, cable, and phone? This means understanding and having used at least:

adb push

adb pull

adb shell

adb logcat

adb remount

as well as shell commands including at least

# logcat

# dmesg

# cat /proc/kmsg

# mount -o rw,remount /system

# mount -o ro,remount /system

Shell redirection to a file, for example, # dmesg > /sdcard/dmesg.today.txt

Using crtl-C to terminate the foreground process in a shell

Yes, "this ain't no iPhone" when you start installing custom ROMs, tweaks, or whatever. You should be prepared to deal with the Linux part of the phone; you're often installing a new operating system and may have to deal with its underpinnings.

No, your GUI-driven "root explorer" really isn't the right tool. You have to be very careful with Windows text editors too. You're lucky I'm not asking you to learn vi.

6) Have you installed and know how to use

Better Battery Stats

aLogcat

aLogrec

7) Have you installed Heimdall, and confirmed that the drivers work for your system (very important for Windows)?

8) Have you already downloaded the "back-to-stock" package for your phone?

9) Have you installed Titanium Backup, preferably Pro, and have complete backups of your system?

10) Have you backed up your Contacts (you can use the phone's Contact app to do this)?

11) Have you backed up your phone logs and SMS/MMS (you can use Backup SMS & Restore and Call Logs Backup & Restore)?

12) Do you have a copy of all of these on something other than the phone and the microSD?

13) Have you read both the thread in Development and the thread in Q&A for the back-to-stock package as well as what you are about to flash? Really, I mean it. Yes, the whole thing. If you don't have time to read the threads, you probably shouldn't be flashing your phone.

If you weren't able to check off all the above. STOP NOW.

OK -- backups in hand, power down your phone. Pull the SIM. Boot into recovery. Do a full backup of everything there as well. This is a "nandroid" backup. (If you're on "stock" without a kernel with custom recovery already flashed, you won't be able to create a nandroid backup.)

Flash the back-to-stock package according to the instructions. Were you able to do that successfully? Did your phone boot? Failure? Read the thread on the back-to-stock package carefully. They really are pretty bullet-proof. You're going to get terse responses if you have problems with the back-to-stock. If you fail here, restore that nandroid backup in your recovery. Go back and make sure you understand how flashing works and re-read the instructions.

Once you can go back-to-stock, make sure you can restore from your backups from steps 9-11. Yes, your SIM is still out, that way you won't be changing anything important on your phone. If you can't, restore the "nandriod" backup. Yes, go back and re-read things again until you understand and can do this reliably.

Made it to here? Go ahead and follow the flashing instructions for your ROM that are in its Development thread, generally in the first couple posts. You did read the entire thread, yes?

Restore your data from backups, if needed. Once you're confident it is running properly, power it down, and now insert your SIM again. Power it up and enjoy your new software.

Have a problem with a ROM or a piece of software? Remember that the problem basically doesn't exist until a dev can reproduce it, or see logs of it happening. Need a hint? See steps 5 & 6 here.