I have an external server that I host BCM on, so that saves the trouble of having to leave my computer on. If you wanna try hosting from your own computer, I'd use a port other than 1137. Use a random number above 30000 or something. In your router configuration settings, look for something about port forwarding. You'll have to forward TCP and UDP connections on that specific port you chose from your router to your specific computer. After updating the settings, you'll need an XML with connection info to your public IP address, or DDNS name.
<wiidisc version="1">
<!-- Mario Kart Wii -->
<id game="RMC"/>
<network protocol="riifs" address="brawlcustommusic.com" port="40000"/>
</wiidisc>
That's what I'm using on BCM right now, but the address can be your public IP.