How do I thread my Singer 7422?

I suggest you do a search on youtube on how to thread a sewing machine and choose a video showing a similar machine as there are really only about 3 basic types of machine as regards the threading.
Some of these instruction videos are very good and you will find it much easier than explanations. However-the important thing is that the thread must go between the tension discs by your tension regulator and do make sure you have a good need le in the machine, inserted correctly.

Incedently, you can purchase a replacement instruction manual from Singer:
To buy a physical copy from Singer:
http://www.singerco.com/accessories/manuals.html