First Things First!
To flash a new ROM onto your HTC Desire, you will need the following;
A rooted HTC Desire.
Your ROM of choice.
A computer, to download your ROM and transfer it to your phone.
Some balls, in case you brick your phone.
Here’s a quick summary of what we’re going to be doing;
Backing up apps and data.
Wiping and partitioning your SD card.
Flashing the ROM.
Backup Apps and System Data
Before you proceed with flashing a new ROM, you might want to back your stuff up so you can restore it after you’ve flashed your device. This may include apps or system settings, in which case you can use a really handy app called Titanium Backup to save all your stuff. Search for it in the app market, there’s a free version which works just fine! Once you’re backed up to the SD card, you’ll want to plug your Android into the computer and copy over the “Titanium Backup” folder to your PC. You’ll be restoring that later.
Wiping and Partitioning Your Android SD Card
Boot your Desire with the volume-down button pressed down. Navigate to recovery. Assuming you’re using the AmonRA system recovery (which you would have installed when Rooting your Desire), your recovery screen should look something like this (but a different color probably);
Go down to the “wipe” option and follow the wizard. Then, use the “partition sdcard” option and follow that wizard too. Specify about 150MB for the ext partition, and use the remaining space as FAT space. You don’t need any swap space either, so set that to 0. Once you’ve partitioned, you will need to convert ext2 to ext3. You can use ext4 if you want, but I think there are some compatibility issues somewhere. Ext3 is fine anyway.
Find a ROM if You Haven’t Already
The xda-developers forum is literally FULL to the brim with custom ROMs for your phone. You can’t go far wrong with it 
I happened to choose the Gingerbread V3 ROM when I flashed my HTC Desire, as it looks minimal, smooth and awesome. I highly recommend it.
Flashing the ROM!
Once you’ve found your ROM, hit the download link and save it to your computer. On your phone’s recovery screen, hit the “USB-MS Toggle” button and plug in your USB cable. Transfer the ROM over to the root of your phone’s SD card. Once you’re done, disable the USB-MS Toggle and choose the option – “Flash zip from sdcard”. Choose your zip file, give it the thumbs up and you can start flashing!
This may take a while depending on your ROM. After it’s finished, you can reboot your phone and start it up. If it’s all gone well, your new OS will load up! If not, then I can’t help because I have yet to break my phone. If something’s wrong, ask a professional or try re-flashing.
Happy hacking!
