add a fuses are pretty simple, it usually takes 5x longer to finda suitable circuit to use than it does to plug one in.
the first thing you need to decide is whether you want to use a switched circuit (only has power with the key turned), or one that's always got power. some people want their dash cams on 24/7, others only want it recording/drawing power when they're driving. a test light or multimeter are the easiest tools to use, but trial and error works too. the last two add a fuses I've used used the windshield wiper circuits for switched power.
pull out an existing fuse that suits your purpose (make sure it's the same size as your add-a-fuse holder or it won't plug in), and plug your add a fuse holder into that empty slot. take the fuse that you just pulled out and plug it back into one of the two slots on the top (should go into the slot that isn't being used for the new power lead, it should be obvious to see when looking at it from what I remember). this re-fuses the original circuit that you're Y-ing off of.
then plug in a second fuse next to it, which fuses the new power lead for your cam.
I can't imagine you need anything more than a 3-5A fuse to power a dashcam. if your dashcam has a fuse size suggestion, use it.