LOL - that last step while reassembling the Mark III can be an easy one to forget and I seem to most every time I clean mine.
My answer, go to the Sturm Ruger website and look through their videos. They have complete dissemble/reassemble videos for most all their guns, including the Mark III
I don't care about Joe Blow's video on YouTube. I go to the source when it comes to my guns.
http://www.ruger.com/resources/videos.html
Click on "Ruger Tech Tips"