Only do this if you have TIME , PATIENCE and another phone with you.
Also… this is not a tutorial. This is only a collection of links, tweaks troubleshoot methods, mostly for myself.
I don’t say this is going to work. If you do anything to your phone, you’re on your own, no one to be blamed, so be aware.

Why?

1 GHz, 576 MB RAM, has more RAM than a usual laptop 5 years ago, but 512MB flash and 150 MB for user is bloat.

Also there are no official, recommended, stable updates for HTC Desire GSM (Bravo) after Android 2.2. They released a semi-official  2.3 update: only for experienced
users – I’ve tried it after a custom ROM, it did not connect to wireless networks, I could not even start a call.

Another reason is that you want to replace the shipped apps, like I did. Sometimes there are far better applications out there than the stock ones – sometimes.

Why not?

Because something will definately go wrong, no exception. For me, it seems I cannot connect USB Mass Storage anymore, no idea why. ( I’ve tried it both in linux and Windows.)

Do not forget to make backups. Always.
And always verify them; belief is not enough.
ClockwordMod backups DO CAN FAIL. 

What you need to understand

The system of an Android phone is made up of a lot .img files. These are byte-by-byte copies of the affs2 filesystems used in embedded devices. My HTC – and probably your device too – has the following parts:

bootloader

Don’t touch this by hand. If you make a bad move, the phone can only be restored by hardware hacks, and you really don’t want to do this. Use  Revolutionary, this will unlock the features of the bootloader (S-OFF), for example, fastboot via USB.

recovery (recovery.img) 

This the “backup” system; an alternative small system you can boot into  to save the day and modify the base system. This system is usually replaced with Amon_Ra or ClockworkMod. Revolutionary installs ClockworkMod automatically.

boot (boot.img)

Loads the main kernel and the system itself. Do not confuse with bootloader: bootloader loads the boot image.

system (system.img)

This is your system, the root partition if it sounds better. Clockwordmod says it makes a yaffs2 backup, but that’s not fully true: it dd-s the filesystem, while it should be cat-ed.

data (data.img)

User data; something between /var and /home.

cache (cache.img)

Cache. Not important.

Tools & resources

• Revolutionary
  Makes things possible.
• HTC developer portal
  You can access the kernel source, but that’s only for really hardcore fans.
• shipped-roms.com
  When you reach the point of crying, this is really handy: they are collecting the original stock roms and installers.
• original HTC Desire system rooted as .zip update ( 2.10.405.5 )
  Origins from a german forum.

Custom ROMs

• cyanogen
  One of the most commonly used custom ROMs: fast, easy to use, quite stable, but has some serious bugs, like the Sleep of Death. Anyway, their wiki is a life-saver.
• LeeDroid
  Some say the best ROM in the jungle; for me, it only threw errors over errors.

Lifesavers

• adb
  This is part of all android systems: imagine it as a backdoor telnet to Android core. It can be used with USB ( default ) or via WLAN, this probably needs a rooted phone and a Terminal Emulator installed. adb can be installed as part of Android SDK.
  Also: http://jonwestfall.com/2009/08/backup-restore-android-apps-using-adb/
• fastboot
  In short, fastboot is the interface to connect your computer to the bootloader via USB. It can re-flash any part of the system except the bootloader itself.
  To see all fastboot oem commands type: `fastboot oem ?`

Hacking

Step 1: Revolutionary

This is the only step without any danger, except you will lose warranty. Do as the site says; no tricks.

Step 2: Backup

Boot into recovery: hold vol down + power, wait, press vol down, wait, press vol down twice, press power, wait for boot.

This was the step that went partly wrong in my case, therefore I’m currently unable to restore the stock system. The thing is that I did do the backup in Clockworkmod but after reboot, it constantly says md5 mismatch. Later I’ve found that it says this for all backups but while most of them is correct ( and can be solved by regenerating md5 hashed, as described here ), though my system.img really went wrong: I’m unable to re-flash it even with fastboot.

Step 3: Flash your custom ROM

This is really easy. Copy the .zip file to the sdcard, boot into recovery, wipe data, wipe cache, wipe dalvik cache, and install zip. That’s all.

Step 4: Wait

First boot takes a long time. The Dalvik ( imagine as JVM for android ) compiles what needs to be compiled. This is stored in /data/dalvik-cache.

Troubles and solvings

Recovery claims: unable to mount sdcard

Get into fastboot and send the following command:

fastboot oem enableqxdm 0

No USB Mass storage

Something happens, but the storage doesn’t show up.

Get into fastboot and try:

fastboot oem eraseconfig

Reboot, try mounting usb. If it fails: http://forum.xda-developers.com/showthread.php?t=1143252

Tweaks

Remove system apps

http://wiki.cyanogenmod.com/wiki/Barebones

Restore stock ROM

Follow the instructions ( Hungarian, so use G translate :) )

http://pzoley.matraszele.hu/desire.php?p=wiki&i=27

And … don’t forget to sleep. It’s important. 

Visit the post at HTC Desire (Bravo): ROMs, updates, pain and suffering