You'll probably get replies with other techniques, but this is what I do and I consider it the simplest method. You will need a microSD to SD adapter for this technique. I'm using my Intercept running Froyo as an example, so you may have some slight differences. Go into Settings/SD Card and Phone Storage, and select "Unmount SD Card". When it says the card is unmounted, remove the SD card from the phone. Use the adapter to interface the card with your computer. Your OS should detect the card, and ask you what you want to do with it. Use the open/view files and folders option. You should now see directories like you would if you were looking at your computer's hard drive. Now, in another window, open your music folder. You can just drag and drop from one window (hard drive music) to the other window (phone microSD) If there's a preexisting folder for music on the card, drop the music files there. If not, create one. Android will find the files. Then just safely remove hardware from computer, insert microSD into phone, and the phone will take it from there. Hope this helps!
Oh, wait. You said Stratosphere. That's VZW. There may be VCast crap to deal with. I hope not. VCast sucks, or at least it did 2 years ago. Good luck!