The next step is to cut off a small chunk of dry ice from the block. You only need about 2 grams, or a piece about half the size of your thumb. There is no harm to putting in too little dry ice - you will simply end up with only slightly fizzy fruit. However, putting in too much dry ice IS dangerous and could make a really big mess.
Dry ice is constantly sublimating
(not melting) from its solid form of CO2 to CO2 gas. Unlike regular ice made from water, it goes directly from its solid phase to its gaseous phase - no liquid phase in between. That is why it sublimates, rather than melts.
As a result, the dry ice block will produce gaseous CO2 until there is nothing left of the solid block. The bottles are going to be sealed tightly with their caps, so if too much CO2 gas is built up inside of the bottle they might explode (the soda bottle bursts at around 115 psi). We are looking for only a little bit of pressure (30 psi) and so there is no need to add in a big hunk of dry ice.
The dry ice in the picture below was enough for both of my bottles of fruit, so each one got about half of the small chips you see below.