OK I have unlocked it but now how do I fit it ?????
Unlocked? If you mean you have the Blueprint, then you have something to do...
If you have at least one Blueprint, and enough Materials (Guardian Materials -
see here, use the Guardian Broker), then you need to find the next Tech Broker for Guardian (use EDDB) and visit him, to unlock.
After unlocking (or if you already have done this) you can buy the modules
on the same station with the Broker in Outfitting. Mind, that you can only buy Guardian modules after you have unlocked them and only on stations with a Guardian Tech Broker, in the future. So if you want to equip another ship, you just return to a station with the right broker and buy the module in Outfitting. You cannot buy the modules in every Outfitting.