Get legally downloaded video files (is there any other kind??!) from friends, family, work mates.
Copy video files to a new Folder on your computer. Name it appropriately. Or inappropriatly if you're so inclined .,,
Add video file to your converter software, and choose MP4 as the file output type. Convert it. Might take 5 to 10 mins depending on size of file.
Find your iTunes files/folders. In Win7 it's under Libraries/My Music/iTunes/iTunes Media. You'll see an Automatically Add to iTunes folder. Copy your new MP4 file to that folder.
Now when you click on Films in your iTunes Library, you'll have an 'unknown genre' - in which you'll have your non iTunes Video Media!! From here you can AirPlay and do all the cool Apple stuff.
Alternatively - I've checked this and it does work - you can just drag your MP4 into this window direct and it'll go into the Unknown Genre folder. Works well when copying one file at a time.
This was my first attempt at a guide and I picked a subject that has been very handy for me lately.. Please let me know if I've missed anything or can improve on it in any way!!