Its two things.
1) remove the large soft rectangular black pad inside the battery cover
2( take your BL-6C, get a nail clipper file and shave a bit of plastic off the four corners of the battery on the side facing out.
Then you put the battery cover on. Dont be shy, you cant really damage it and you will need to push and squeeze. It does stay on fine, although a tight fit.
For all your troubles you get a piddly 10-12% more battery life, but the phone feels slighly heavier tho. I've done it myself.
Methinks buying an extra BL-5C + charger, loading the battery and putting it in your pocket/bag is probabaly a better move. Unless you just want the satisfaction of a pretty basic and un-impactful "mod".