Shallow old work box15 amp duplex receptacleCut or pull wire from existing receptacle. Install box with wires through box entrances. Attach wires to new receptacle or establish pigtail-3 wire from existing wires to new receptacle. Install new wires from extension of branch to new receptacle.
This is one of the most basic, easy projects for DIY electrical work and you'd be surprised how often it's done wrong. Phases are 180, ground not connected, wires stripped too much....I could go on. Just look up the basic instructions on how to connect and don't overload the existing branch with too many higher watt devices.
BTW, learning electricalese, the language, is something to try for. A plug is a male cord end. A receptacle is what the plug goes into.