Here are two videos that clearly show the procedure for changing the oil (and filter) on you WR four stroke weapon.
The video shows a wr250, but the procedure also applies to the mighty WR450 as well.
It is worth noting that the guy has a reuseable stainless steel filter (similiar to what Ollie is selling) installed on his bike. He goes through how he cleans it, which may be helpful to all of you who have recently purchased this style of filter.
Part 1
http://au.youtube.com/watch?v=grOfpX-DRMM
Part 2
http://au.youtube.com/watch?v=Xqj1W38qZfg&feature=related