I've made a few ringtones and other phone sounds before. But it doesn't always work, it depends on the phone and the carrier. Here is how I do it:
1. Hope to hell you can find the sound you want in a good quality youtube video. Heck finding the sound in any actual media file is good enough.
2. Convert said media file into an MP3 or WAV (depends on the phone).
3. Take photo with the phone, and send as picture message to email account (note, this doesn't work as easy for AT&T phones, easier for Verizon)
4. Reply to email, but remove everything (even signature!) and attach converted file.
5. On the phone (Verizon) it will appear as "Picture message", but when you open it, it will play the sound. Under the options area you will see one to save as tone or something like that.
6. After it is saved, assign it as you please.
I've made all the ringtones and alert tones for my phone and also made/sent tones to other people. As noted above, I've had most success with Verizon Phones. AT&T phones can sometimes be a problem and/or require a lot more work to get the tone working.