I'd just like to add some things to save someone else some heartache.
When you are running the cables up MAKE SURE that you run them back through the holes in the bike frame. I forgot about this and had to snip and reconnect them again. If you dont run them through the holes the undertail will not fit properly.
You dont need to do any cutting i.m.o. I have a 2011 ninja 250 and the undertail just slots on over the battery box plastic part.
Pain in the ass getting the lower bolts back in. I only managed to get one in.
Definitely screw out the reservoir cap screw as mentioned above, this will get in the way and scratch your undertail if you dont.
Make sure that you buy connectors aswell, you'll need about 10 or so.
Its handy to have someone to help you with installing for certain parts.
Getting the bolts removed from the tail light stock is a challenge without the right spanner, I would recommend getting one Im not sure of the size though, I used a vice grip but it wasnt straight forward.
Tools needed:
Pliers (very handy for electrical work as you can snip and also clamp down on the connectors to close them)
Set of allenkeys
Small flathead screwdriver and medium sized head flathead.
Drill with a size 8 steel bit.
Patience
