In short... Don't!
Never ever mix cells of different chemistry for anything than a short test run where no other solution can be found.
Neither should you mix cells of the same kind, but with different capacity.
The weakest cells will discharge faster and then recharge in reverse, re-poling the cell(s) in question, which is a semi-dangerous way of destroying battery cells, as the possible scenarios are... Weak cell, dead cell, leaking cell or just a quick little *kaboom* (a.k.a. No cell at all, but a nice pattern on the wallpaper except where you were standing [sort of like running over a turd with a fast spinning lawn mover]).
(Sure, I like to exaggerate when it comes to unsafe behaviour of the third kind
If you really want dead cells, hire the Energizer Bunny - it's much safer (although he might be a tiring employee going *twang twang twang* for days on end