How do you turn a single shot assault rifle into an automatic?
There is no such thing as a single shot assault rifle. By
definition, all assault rifles are full-auto. There is a similar,
confusing term, "assault weapon", which only applyed to semi-autos.
Semi-autos only fire a single shot per trigger pull.
Adding to the above, if you're asking how to turn a semi auto
into a full auto, you must first obtain a Type 7 or Type 10 Federal
Firearms Licence, with a Special Occupational Tax category 2 stamp.
Manufacture of automatic weapons without licencing to do such is a
felony, and a how-to guide will not be provided here.