At Navyaviation.com you'll find a lot of information about your Singer 111 W 151.
Firstly, check out the picture on this page with all the parts to your 111W151. Once you feel you've got yourself acquainted with the machine's parts then head over to this page for a step by step guide on how to thread the machine.
Click the blue back and forth buttons at the top of the page to get a lot more information on your machine: How to replace the bobbin, how to regulate the length of the stitch and also how to lubricate the different parts.
If you'd like to download the manual as a PDF for safekeeping then got to page 15 of this document from DesignerEntrepreneurs.com and read or save as you wish.
Google has a free ebook download of the April 1965 government document by the Department of the Army "Sewing Machines for the Repair of Parachutes and Allied Equipment."
Here's a cool video about the different features of your machine from a Youtuber.
And just as an extra, here's some history on your wartime industrial machine http://nwtarp.blogspot.co.uk......