when installing led’s in bit shells use the smallest possible, 3mm should be the max really. surface mount are the best, try shopping at jaycar electronics or a similar shop. as for getting them in, any way you can really. if you’re using button cells then just wire them directly on. anything larger and you’ll have to use resistors and maths! here’s a pic of some of my light installations

http://www.geocities.com/bettyk3332003/attachingshellstute.html
i usually paint them black or cover them with black tape so the light only comes through where i want it.:8ball: