The main stages are:
1. Remove the keyboard.
2. Lift out the PC board.
3. Replace the drive.
4. Reverse to reassemble.
1. To remove the keyboard, begin by removing two screws on the sides, one next to the power switch, the other next to the infrared. Next, release the keyboard from the case by gently easing it up with a small screwdriver while simultaneously gently levering the case outwards (away from the keyboard).
Once you have the sides and front free the keyboard will release from the back as well as you pull it up. Do not pull far as it is still connected by four cables. Unplug these. The small flat cable connector (for the pointer) has a brown fastener that lifts up to release. The big cable (for the keys) has a white fastener that pulls towards the front to release. The other two two-wire cables are for the stereo speakers.
2. To lift out the PC board, first remove the following nine screws securing it, There are two long black screws underneath the case in the middle. On top, remove the CPU heatsink (three medium-length silver screws), the two short black screws at the right hand corners of the board, the short black screw in the middle of the white plastic piece in the middle of the board, and the short silver screw securing the PCMCIA chassis to the case bottom, just inside the front edge towards the left low down. That's the nine.
Now remove three cables: the two video cables at the back (these just lift off), and the parallel port cable on the right. The fastener for the latter is of the lift-up (as opposed to pull-out) kind: after lifting it pull the cable to the right.
The remaining nontrivial things holding the board down are a plug underneath towards the rear connecting it to the power supply at the back, and the backup battery in the memory lid underneath. The backup battery can be slid out of the memory cover. Put the cover to one side while leaving the battery plugged in (but take care not to yank its cable).
To detach the board from the power supply, lift it up at the back middle. I was always able to lift it off working from the top, but it occurs to me that I could have removed the memory cover and pushed on the base of the memory board (so as not to put pressure on the memory module).
There are still two trivial things keeping the board stuck. (i) The bracket at the from from which you removed the silver screw tends to catch on the case, pushing it back a little from the case frees it. (ii) The PCMCIA eject buttons and the power switch must be cleared, done by lifting the right end of the board to clear the case and then sliding the board right a little, then lifting the left end.
BE VERY CAREFUL WHEN LIFTING THE BOARD not to let it fall back onto the heat sink in any but the correct position, or to let anything else touch the CPU on the underside of the board. The tiny pins on the CPU are very delicate and if you smoosh them against the heat sink bolts you can short them against each other. They are really tiny, about 250 microns wide, 100 microns thick, and spaced 500 microns apart. I made just this mistake and bent half a dozen and tore one right off its trace. Fortunately I was able to repair all the damage with the help of a high power magnifier, a microminiature soldering iron improvized from a length of 26 ga wire sharpened to a needle point and wrapped around the bit of a regular iron, and a hand steadied by a night's sleep. It was touch and go and if I'd damaged many more pins I'd have had to visit the med school to see what microsurgery tools they had.
3. To remove the hard drive, remove the four large screws holding the drive to the board and unplug it by sliding it to the right. Install the new drive.
4. To reassemble, reverse steps 1-3. Hints (besides the mandatory care with the CPU): lower the left end of the board into the case and slide to the left to engage the eject buttons and the power switch. Push down on the back to plug the board into the power supply. The rest should be obvious.
One other detail. There is a hole underneath the IBM drive that you are instructed not to cover. I first considered drilling a hole in the case aligned with that hole, but then I decided to see whether there was room to put a spacer giving the hole some breathing room. So I cut out a 5mm square piece of rubber from a (deceased) bicycle tube and glued it to the drive about 5mm away from the hole, hoping this would lift the drive far enough off the bottom of the case for the hole to breathe. There was no problem with reassembly so apparently there is enough room.