i prefer to make it appear stock. I need to know how to open the fuse block and add a circuit in there.
If I understand you correctly you want to add a circuit into a blank spot using one of the two factory fuse box locations?
If that is the case you need to have access to the back of either one of those fuse blocks. You are going to need to be able to size and terminate whatever type of end or connection that OEM block accepts. You are going to end up disturbing a lot of stuff just to do this.
What are your power requirements? In amps.
What are you trying to run or control?
How are you going to switch that load? Relay, switches, ignition controlled?
Its a lot easier to just tap off an existing circuit depending on your needs. For example a low current device like a radar detector, 12+ for relay control, perhaps a USB charger... Using something that looks like this to share an existing circuit.
For larger loads I think you will have more success and flexibility just adding your own AUX panel into the cab and leave all the OEM stuff alone.
I added one in my passenger side quarter panel area... it feeds all my inside accessories.