Xcode

First, download the installer

MacOS 10.7

Mount the installer, then in Terminal, copy and paste these lines:

export COMMAND_LINE_INSTALL=1
open "/Volumes/Xcode and iOS SDK/Xcode and iOS SDK.mpkg"

MacOS 10.8 and later

You have to be paying VERY close attention during the install, or you will bluescreen your mac. Yes, a kernel panic so you can't even boot! See below about "kernel panic" if you get stuck there. Read the entire list BEFORE starting so you know the steps AHEAD of time.

1. copy the "Kernel Panic" section below to a plain text file on your desktop. You'll be glad you did if things go south.
2. Mount the installer
3. Go to /System/Library/CoreServices
4. Get info on "Installer"
5. check "Open in 32-bit mode"
6. Go to /System/Library/Extensions
7. start to type "AppleProfileFamily.kext", and your selection will then land on the existing .kext by that name, OR the next file alphabetically.
8. if it exists, trash it. you may need to enter your PW for that.
9. Quit iTunes if it's running
10. open "Activity Monitor" and kill anything relating to "iTunes"
11. During installation (after next step) you must watch for the file "AppleProfileFamily.kext" appearing and IMMEDIATELY delete it. If you wait too long and it starts to run, you will get a kernel panic. (see below in case of kernel panic)
12. Paste this into terminal:

    export COMMAND_LINE_INSTALL=1
    open "/Volumes/Xcode and iOS SDK/Xcode and iOS SDK.mpkg"

13. the cert has expired, continue anyway
14. when you get to the "Installation Type" screen, click "customize"
15. turn OFF "System Tools"
16. turn ON "Mac OS X 10.4 SDK"
17. Twirl down "Essentials" and turn off "iOS SDK"
18. start the install
19. watch the Extensions folder like a hawk, do not take your eyes off of it: the instant you see "AppleProfileFamily.kext", select it and do command-delete! note you may need to type your password real fast too!
20. empty the trash! QUICK!!
21. relax, and wait for the install to finish
22. switch "Installer" back to NON-32-bit mode
23. you're good!

In case of kernel panic

1. hold down the power button to shut down
2. power on and immediately hold the SHIFT key (safe mode)
3. when you get to the finder, go to /System/Library/Extensions
4. trash the file "AppleProfileFamily.kext"
5. if you accidentally also installed the "System Tools" or if CHUD somehow got installed, then you'll need to throw that stuff out too. eg search for "com.apple.iokit.ChudKernLib" and chuck all the stuff with "chud" in it that's nearby.
6. empty trash
7. reboot
8. you're good!