How to make an mp3 music into your iPhone 4 as ringtone?

To be specific, I know you have to choose 30 seconds on what your desired mp3, then cut it, make it into another file. But the question from that point is, I have to play it to add it into my phone, but when I did, that file is not showing. Moreover, it says I have to sync it, so is that mean I will have to lose all my music just to import a 30 second song as ringtone?

Step 1
Open the iTunes application and locate the MP3 file you wish to set as your ringtone. Play the song file until you find a 30 second clip that you wish to set as your ringtone. Take note of the clip's start and stop times.

Step 2
Right-click the song title and select the "Get Info" option from the context menu. Click the "Options" tab and place a checkmark next to the "Start Time" and "Stop Time" options. Enter the start time and stop time you noted in the spaces provided and click the "OK" button

Step 3
Right-click the song title and select the "Create AAC Version" option from the context menu. Allow several seconds for iTunes to create a new version of the song.

Step 4
Click the new 30 second AAC version of the song and drag it onto the computer's desktop. After the file has copied to the desktop, delete the file from iTunes. Right-click the song file and select the "Get Info" option. Click the "Name and Extension" subheading and change the file's extension from ".m4a" to ".m4r."

Step 5
Close the "Get Info" window and double-click on the new ".m4r" file to add it to iTunes. Plug your iPhone into the computer and sync it with iTunes.