Total novice - 6mm alan/hex key on a ratchet?

So I've got this set from screwfix.

https://www.screwfix.com/p/715yc

Better image: https://postimg.cc/xX3sVBYm

I'm having a hard time working out how to connect a 6mm hex/alan key onto the ratchet.

Again, very novice at this.