So I have tons of experience working on bikes, and by now know a lot about compatibility of various parts, but there are some sort-of newer parts that I've never actually worked on. Such as integrated crank/bb systems, threadless steerer systems, and Shimano STI levers. (My bikes have all had downtube shifters or barcons, because I like the philosophy a bit better and for durability, which was more of an issue when I built up my last nice bike in 1995. I have worked on bikes with Campy Ergo levers.)
I'm currently working on a friend's bike, a Cannondale from the early 90's that has 8-speed Ultegra STI levers. I'm giving him a Technomic stem, so I have to remove the levers from the handlebar, and can't figure out how to do it. I've not searched around too hard, but it doesn't work like standard (old-school) brake levers, aero or not.

I'd like to hear how you remove STI levers from the handlebars, as well as how you install shifting cables. Brake cables are easy to spot.
here's Shimano's technical diagram of the current 10-speed Dura Ace levers, if it helps for reference in explaining things. Thanks.

https://parktool.com/repair/readhowto.asp?id=114

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.

Actually, the shift cable is a piece of cake. All you have to do is to shift into the smallest sprocket position and it pushes out of a little window opposite of where it enters the shifter.

The brake cable is the one that you have to pull the brake lever to remove. For your first time my advice is to do one side at a time so you can copy the other side if you get confused.

Removing the levers requires a 5mm allen wrench. There's a little groove on the lever body that you can see on the outside of the lever near the top of the rubber hood.

If you're just removing the brifters to swap a stem, there is no real need to remove the cables after you remove the handlebar tape.