The Park Tool article is decent. What it doesn't mention is making sure you shift the front and rear derailleurs to the slackest cable position else the shifter cable anchor point may be rotated into and hidden by the lever body when you go looking for it. For most derailleurs, shift the bike so the chain is on the smallest chainwheel and the smallest rear cog before disconnecting any cables.

As for the details, all I know about are the 9 and 10sp STI levers of recent vintage. If your early '90s levers are the same, just squeeze the STI lever as if applying the brakes and the shift cable anchor point is exposed on the top. It should be obvious where it is if you note where the shift cable enters the lever assembly.

As far as finding the anchor bolt, you have to go in from the front of the lever and peel the rubber back as far as you can to see the hex bolt. I think it takes a 5mm hex wrench but don't quote me. The bolt is black so if you are working in dim light you may have to feel around with the wrench to locate it.