After having rooted the phone, installed Android SDK and Custom Recovery on your phone, installing a Custom ROM is as easy as copying the ROM file to the root of the SD card and flashing the zip from it.
If you're worried it'll void your warranty, don't be! You can go back to the original LG firmware whenever you wish : How to flash Original Firmware on the LG Optimus One P500.
If you have any problems with Custom ROMs (installation, usage etc) then post them here : Problems related to Custom ROMs
Here is a list of some great ROMs developed by people at xda forums.
- void ROM (customization at its best!)
- AmberHome v1.1
- Prime v2 (My favorite, I'm currently on Prime v2)
- DarkMoon v01
- Megatron v1 (pure power)
- Droideka v2
- Ioshirom v2
To flash a custom ROM(make a backup if you want to keep apps, else you can always download them from the market) :
- Boot into Custom Recovery
- Wipe your cache, dalvik cache, SD card
- Make partitions according to the ROM requirement
- Wipe first 3 options of wipe menu again
- Toggle USB MS and paste the ROM zip file to the root of the SD card
- Disable USB MS
- Select 'Flash from zip' option in Custom Recovery menu
- Select the ROM file
- It'll take sometime and reboot into your new Custom ROM!
In case you haven't rooted your phone, here you go: Rooting LG Optimus One P500
In case you haven't installed the Android SDK, here you go: How to install Android SDK /ADB?
In case you haven't installed the Custom Recovery, here you go: Installing Custom Recovery on LG Optimus One P500