How do I splice wires in the attic to install a plug socket

If the attic is unfinished, odds are there are several junction boxes in there. If you are using a light that is going to be on a pullchain, then you need to find a hot and neutral in one of the junction boxes to take to the light and the receptacle. If it is going to be switch operated then you need to run the hot and neutral to the switch, and run a line to the light. The two neutrals will just tie together in the switch box and not go to the switch. The hot feeding the switch will go to one screw on the switch and the other wire (switch leg which should always be black) will go to the other screw on the switch.

I want to re-affirm that any splices must be contained in a junction box with a cover, and must be accessible.

Yes good call tk. And the light itself needs to mount to a box also.