I rooted my LG GT540 now I want to install custom rom but don't know how to do, so I am looking for some easy steps to do this and also share which custom will be best for this device?
Now you have set up your phone to connect to your computer correctly. From here on, it depends on which ROM you wish to install. This is an excellent video tutorial that covers installing custom ROMS and rooting, and as said in the UPDATE, many ROMS with instructions can be found on XDA.
Bricking is always a possibility when modifying software that doesn't come directly through the intended channels
That said, the methods here have been proven to work and your risk is significantly less if you perform these steps correctly.
Let us know if you need any help!
UPDATE: this is a method to install updates to your phone. as far as specific ROMS to install, XDA and google can be very helpful. As soon as you find one you like, go for it :)