I tried to install CHDK on a Ixus SD 300/Ixus 40.
It took about four hours and a lot still not work.
Every missing hint costs between 1 minute and 1 hour.
First: Extract the files to the ROOT DIRECTORY of the card (Important: ROOT DIRECTORY, not DCIM)
Second: Select Play mode, Press and hold FuncSet (in the middle of the ring), then press Disp (left lower button).
Third: SWITCH THE CAMERA OFF (important, otherwise you do not see the firmware update menu entry), then switch the camera on.
Fourth: Replay Mode, the Menu, then Play (|>), lowest menu item is "Firm update" (This is new!). Select this and press okay.
Fifth: Press the direct print button, the the menu button to enter the alternative Menu.
Unclear: How to make the firmware auto bootable. Misc => Make card bootable flickers, but generates no message and has also no effect. After switching off the camera you have to do the whole stuff again.
Unclear: DNG can not be selected. Bad Pixel list is empty (but exist) after running the badpixel script.
Cannot load CHDK/badpixel.bin Please run CHDK/.../badpixel.lua
Unclear: There's no program to handle Isus SD 300/Ixus 40 RAW images.