Hmmm no kids for me to teach anything to yet and I'm all prepped after half term so let's see what I can remember...
Bt is a toxic bacterium. If it makes it into an insect's gut then it produces a protein (known as Cry) which basically increases the porosity of the insect's intestines (i.e. the insect melts from the inside).
Cotton is a plant that we like to grow that is regularly attacked by insects
Bt cotton is cotton that has been transformed with the CRY gene from Bt, which allows it to manufacture this toxic compound.
Because the cotton is then protecting itself we need to use far far less insecticide (cost effective) and only insects that eat the cotton are targetted, rather than all of the nice insects living in the field but not attacking the cotton.
Now... as for how it gets in there.... IIRC cotton isn't recalcitrant so what they probably do is take the CRY gene out of the insect's DNA, make it into a plasmid, infect Agrobacterium tumefaciens with the plasmid, and then let it lose on some cotton. (Agrobacterium will naturally attack plants - it causes crown gall disease - and it incorporates its DNA into the plant, a bit like HIV).
Of course, I could be wrong, in which case you'd take really fine gold pellets and basically soak them in DNA and then bombard the plant with them with a high speed gun. But that's expensive, so we don't like doing it