It's quite straight forward. Make sure both the phone and computer are connected to the same wifi network. Open windows media player on your computer and under the streaming tab there should be a setting allowing 'devices to connect via the Internet' something along those lines, sorry I'm not in front of my computer. Now on the phone when you view a picture or play a video using the stock apps there should be a wifi-ish icon, tap that and it'll transfer to the windows media player. Should be available in the music player app too but I haven't tried that.
Sorry I think I've explained that pretty badly lol. But try it and if you get stuck I'll help you out later when I get home from work.