Well, honestly I wouldn’t even bother. Unless, of course, you care nothing about sound quality. WMA, MP3, AAC, OGG, and a number of other formats are what is known as Lossy formats.. essentially the idea behind lossy formats are that they assume that there are certain parts in an audio stream that the human ear cannot detect (e.g., very high and very low frequencies) and so to save on file size, removes these pieces of the audio based on the format’s specific algorithm. When you start with a high bitrate source, like a CD, then convert it into a lossy format it sounds pretty good most of the time. But, then, if you would convert a lossy to another lossy, you would be starting with an already lowered bitrate, which would degrade further as you convert. Not only this, but WMA and MP3 use different ways of determining what can be cut from the file.. converting one from the other will only make your file sound grainy or even garbled.
Another way to look at it would be as if you were to make a photo copy from an original (i.e., your WMA) then make a copy from that copy (i.e., the mp3 you want). It wouldn’t nearly be as nice as if you were to make a fresh copy from the original.
If you have a lot of music, I’m sorry to break the news of this to you, but I would just re-rip it from your collection. Or, optionally, re-download everything with bittorrent as most files that are available are already in mp3.