Change your Android(2.3.4) BootScreen and make it your own Phone

Let's face it: as Android users, we like options. One of the greatest things about this platform is the insane level of customization possible, especially if you don't mind getting your hands a little dirty. With some readily available tools (all of which are extremely free) and the proper knowledge, you can make your android phone do almost anything you could possibly want and make it look however you want. What we'll be talking about today is the bootscreen.
The bootscreen is that animation that plays during your phone's (admittedly long) power-on sequence. It's really easy to switch it out and, provided you've found one you like, I can show you how to change it. After some deliberation, I've decided the best place to start is the beginning.
There are some tools you'll need to have installed before we can start. First off, except in some special cases where the /system/ directory has write access already, you'll need root access. Chances are you can find out all about that by heading over to XDA-Developers and searching for your device in the forums. Once you've done that, you'll need one of two things:
A file explorer on the phone that supports root functions. Some popular ones are Root Explorer, SU File Manager, and ES File Explorer.
The Android SDK installed on your computer. To install the SDK, follow these instructions:
Download and install the Android SDK. This should also install the USB drivers you'll need.
If it doesn't install the drivers, then follow these steps.
Plug your phone into your computer.
Go to the settings menu. Navigate to Applications>Development>USB Debugging and check that box. 
Using the command prompt on your computer, navigate to the "Platform-tools" folder of the SDK.
Type in "adb devices" (without the quotes) and hit the enter key. If your phone shows up (usually a crazy string of letters and numbers), then you're good to go.

Now let's say you have found a bootscreen that you really like and that matches your phones resolution (that's important). Sometimes they'll package it as a handy flashable .ZIP file that you can simply apply in recovery mode. Most of the time, though, you're going to download a bootanimation.zip file that will have to be installed manually.
TL;DR;
Basically, the idea is that we are going to be putting a new animation on our phone that will run instead of the stock bootscreen and, if we can't manage that, we'll replace the old one with a sucker-free one. We'll do them in order of difficulty. (Bonus points for you if you caught the reference in this section's title)
It is very important for me to note here that, for the purposes of this tutorial, everything is going to be geared toward the HTC Evo because that is the device I'm currently using. It has been brought to our attention that there are some variables across devices. The location of the file may be different on your device.
Most devices seem to store their animation is /system/media/
My HTC devices all store bootanimation.zip in /system/customize/resource/
Some users have reported being able to load bootscreens into /data/local/ which is significant because /data/ is always writable and does NOT REQUIRE ROOT ACCESS. This has not worked on my EVO, Hero, or my friend's Droid Incredible.
If there are any other locations that work or don't work you can let us know in the comments and we'll incorporate them (thanks to all the people who helped me out with this information).
The Root File Explorer Method
This is, by a wide margin, the simplest method and in no way requires a computer. You can download the bootscreen directly to your phone and move it where you need it all on-device.
1. Install The App
Download and install one of the above mentioned root file explorers (Root Explorer, SU File Manager,ES File Explorer) or any other of your choosing. It just has to get root access.
2. Download your boot animation.
I mean...yeah. That's the point, right?
3. Backup (OPTIONAL)
You can either copy the original bootscreen and paste it on your sdcard/computer, or you can rename it bootanimation.bak and leave it in the original directory for safe keeping.
4. Copy/move
Copy the bootanimation.zip to the directory that it belongs in.
5. Reboot. Enjoy.
Reboot. Enjoy.
The Recovery Flash Method
The flashable .zip method is as straight-forward as straight-forward can be:
1. Put the .zip file on your sdcard
2. Boot your phone into the recovery console
3. Select "Flash ZIP from sdcard"
4. Find the .zip file
5. Flash it
6. Enjoy.
The ADB Method (AKA The "Aw, Dang It" Method)
For each phone the location may be a little different, but the idea remains the same. We're going to be swapping the bootanimation.zip that came on the phone with one that doesn't suck. I'll be using a Windows computer to do this on my HTC EVO 4G.
It's pretty simple if you're comfortable with ADB. Even if you're not, though, it's simple enough for you to pick up quickly
1. The Hunt
Find your animation. There are a multitude of places to find them, but I plan on showing a lot in the future, so let's just assume you want this one.
2. The Name
Rename the .zip file to "bootanimation.zip".
3. The Location
Place this .zip file in the same directory as the SDK's "ADB.exe" file. If you have a recent version of the SDK, this will be in the "platform-tools" directory. On older versions, it will be in the "tools" directory.
4. The Commando
Here's where the magic happens: We're now going to the command line or, as I call it, the commando line. That sounds way cooler.

5. Black Ops
Now, we'll navigate to the directory with ADB in it that we talked about earlier. (My Android SDK folder is named "ASDK" for the sake of brevity.)

6. Did You Plug It In Right?
Now we're going to run a command to make sure your phone is being recognized by your computer. Type in (without quotes) "adb devices" and hope that some crazy numbers and letter show up.
7. This Is Getting Real
Neat. We're ready to roll. We're now going to mount the system as read/write so we can mess with system files. My favorite method is the simple command "adb remount". Alternatively, you can type in "adb shell" and (on the EVO, as well as most other HTC devices) run the command "mount - o remount,rw -t yaffs2 /dev/block/mtdblock3 /system" then, subsequently, "exit". Well done. You are now the builder and destroyer of Android (provided it told you 'remount succeeded').

8. Back That Thang Up
We're in the home stretch now. Remember how we put the bootanimation.zip in the folder we also have ADB in? Here's why. We are going to back up the old bootanimation.zip then install our new one. The first order order of business is getting into the shell (basically using the command line in the phone using our computer as the window to see it). Type in "adb shell", hit enter, then type "su" and hit enter, just to make sure you're in superuser mode. Then we'll navigate to where the current bootscreen is on the phone. Mine is in /system/customize/resource/ so I'm going to type in "cd /system/customize/resource" then, just to make sure I'm right, I'll type in "ls" and verify the file is there. Now, let's make sure we have a backup in case we do something ridiculous and need to roll back. Type in "mv bootanimation.zip bootanimation.bak. Basically, the old one will sit there and, should we need to, we can rename it back to bootanimation.zip and it will be back to its old self. Issue the exit command until you're out of ADB shell.

9. Do The Deed!
This is it, guys. The final step. We will now issue the magic command:
"adb push bootanimation.zip /system/customize/resource/"

10. You're Done. Turn It Off And Enjoy.
The name of this step is also the entirety of the step. You did it.
While it may seem a little convoluted, this project is easy for beginners and SUPER easy for advanced users. It makes an awesome introduction to ADB and Android modding in general. I hope this helped you out and I look forward to seeing how all you crazy cats use and abuse this information to do hilarious, amazing, and entertaining things.