Normal set base = two normal sized skins stuck together giving you an extra long skin. You do this by lining the two skins (with a slight overlap in the middle) up and then sandwhiching a second folded skin in the middle so the gum (which needs to be licked) is touching the inside of both skins on the overlap. leave the damp skin in for a few seconds then remove leaving the gum left behind (gum transfer) and giving you double the lengh of skin as you prevoiusly had.
A knee trembler = 2 single regular skins joinded together by sticking the gum from one of them onto the other giving you an extra fat skin. Then lick half of the gum on this "fat" skin and join it at a 45' angle to another single, making sure the gums on the same side, and their you go. Hope this is a decent enough explanation I found it hard to explain with out being able to show you