Basically all of the pieces come on these little frames usually referred to as "runners" and you have to carefully cut them out. I recommend some cutters that kinda look like this, they make the process WAY easier
Anyway, the instructions will tell you what pieces to cut out for each step. For example if it says B13 then you go to the runner labeled B and find part 13
The nice thing about the calibarn is that it tells you which runners you need at the start of each step
One of the things I both love and hate about building Gundam models is the sheer amount of detail you don't notice until you've built it. One the one hand it makes it feel like it looks better in person, but on the other it makes you realize how long it'll take to panel-line