On the IBM ThinkPad 380D laptop the CMOS batter for the BIOS is located under the RAM cover on the bottom of the laptop so (after first removing the power cable and the main system battery) you then need to remove the RAM cover found underneath the laptop:
As you can see from the above photo, the CMOS battery is hiding next to the system RAM, it's held in place by some double-sided tape so to remove it all you have to do is unplug the lead and then carefully pull the battery away from the laptop.
You now have 2 options; Option 1: Look on eBay for a replacement or Option 2: Make your own replacement...
If you chose option 2 then read on...
Now you have removed the CMOS battery, you now need to use a knife or other sharp object to carefully remove the plastic surround of the battery to reveal the terminals connected to the battery:
Using some pliers, very carefully remove the terminals from the battery so you are left with this:
For the next part you will need some heat-shrink tubing, some conductive tape and a replacement CMOS battery.
Using the conductive tape, secure the red lead to the positive side of the replacement battery and the black lead to the negative side of the battery.
Next you need to cut a small piece of heat-shrink tubing to size and place over the battery.
Using a heat gun (or a soldering iron held very closely) shrink the heat-shrink around the battery - the reason for doing this is to prevent the battery from short-circuiting to any of the metal parts of the laptop.
Now all you need to do is insert the replacement CMOS battery back in to the laptop: