Minecraft wiki entry for Advanced, associated text: 1. Create a folder wherever you like, name it with your intended pack's name.
2. Jump into that folder.
3. Create a new file manifest.json with the following content:
{
    "format_version": 1,
    "header": {
        "description": "[Your pack description]",
        "name": "[Your pack name]",
        "uuid": "[Pack UUID]",
        "version": [0, 0, 1],
        "min_engine_version": [1, 2, 6]
    },
    "modules": [
        {
            "description": "[Your pack description]",
            "type": "resources",
            "uuid": "[Module UUID]",
            "version": [0, 0, 1]
        }
    ]
}