No, you don’t need a citadel or POS. You can do this in normal stations.
Basically what you do is make a copy of the original blueprint, and then use the copy along with Datacores to invent the T2 blueprint. There is only a certain % chance each run will succeed.
If you look at the BPC, it’ll list the datacores needed for that specific blueprint under the Invention tab. (For Scourge missiles it’s one Electronic Engineering and one Rocket Science datacores.)
You can also, optionally, use Decryptors when inventing, to change the efficiency modifiers, run limits and success chance.