Step 1. Obtain a .BRSTM. These can be found on such sites like
Brawl Custom Music or by converting your own music files yourself (see the tutorial linked above this post for example).
Step 2. Rename .BRSTM according to the song you wish to replace. You can find the naming guide attached to this post.
Step 3. Place the renamed .BRSTM in the "STRM" folder on your sd card (*sd root*>projectm>pf>sound>strm)
I'd hardly call that the "quick" method, especially when you're telling the poor guy to utilize a bunch of probably foreign programs like it's nothing.