Thinking of converting this old fella into a single speed. How should I go about doing so if I wanted to keep as much of it stock as possible?

Take off front and rear derailleurs, shifters and cables, ditch the big and small chainrings on the crankset, you’ll probably need some spacers or shorter chainring bolts to make sure they hold the middle chainring properly without the big ring taking up space, find what size cog is gonna work best with the middle chainring to give you the optimal gearing for you, remove old cassette, then get an appropriate single speed cog and spacers, position the cog on the freehub body so that you have good chainline (i.e. straight), affix cog and spacers on freehub body with the cassette lockring, boom, singlespeed. This assumes you have a cassette and freehub body not a multi-speed freewheel. If it’s a freewheel it can be trickier.