Download Bitpim, then upload the songs you want, you will be able to create mp3 ringtones from there by editing the mp3 then saving it. AFterwards just transfer the file to your phone.
Recently when i was at Alltel buying my LG8600, I also purchused a memory and music bundle that came with headphones, a memory card reader and 512mb memory card. The man that helped me said that I would be able to download my own MP3's from iTunes and use those as ringtones on my phone. I have been able to get the MP3s from my computer to my phone, and can listen to the songs on the phone's MP3 player, but I can't figure out how to make those my ringtone. Can anyone help me?