While scanning someone with it isn't very difficult, actually getting your hands on one is.
Where to Find a Recon Scanner
Finding a Recon Scanner in Fortnite is somewhat down to luck. You can find them as Epic and Rare varieties in chests scattered around the map, but they're pretty rare.
You can thank Epic for making them powerful for them being difficult to find. Keep searching and keep your fingers crossed that you find one in a chest or from looting and enemy you've killed.
The only sure fire way to get yourself a Recon Scanner in Fortnite is to search inside an I.O chest. They almost always contain one, and you could also find a Kymera ray Gun inside, making you powerful in a fight.
How to Spot an Enemy Player with a Recon Scanner in Fortnite
Once you've got one, wait until you're near an enemy - it's best when you're close by and they're unaware.
Then, aim the Recon Scanner near to where they are and shoot the projectile, which will start the scanning of a nearby area.
If you've done it right, the area that the projectile scans will mark the enemy for you and you'll be able to approach them to kill them. You'll only have two charges, so be sure not to mess it up.
It's worth noting that you don't actually have to kill the enemy you scan to complete the Week 7 Epic Quest. All you need to do is mark them, so don't worry too much about doing it in a sneaky way if you're only worried about the challenge and getting that XP.