My custom arrow key mappings. The right option key is used to 'shift' into a different mode.

ninja.json

{
  "title": "Arrows (right_alt || right_control) v0.2.0",
  "rules": [
    {
      "description": "right_alt + e = up",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_alt"
              ]
            },
            "key_code": "e"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "up_arrow"
            }
          ]
        }
      ]
    },
    {
      "description": "right_alt + d = down",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_alt"
              ]
            },
            "key_code": "d"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "down_arrow"
            }
          ]
        }
      ]
    },
    {
      "description": "right_alt + f = right",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_alt"
              ]
            },
            "key_code": "f"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "right_arrow"
            }
          ]
        }
      ]
    },
    {
      "description": "right_alt + s = left",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_alt"
              ]
            },
            "key_code": "s"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "left_arrow"
            }
          ]
        }
      ]
    },
    {
      "description": "right_alt + t = page up",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_alt"
              ]
            },
            "key_code": "t"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "page_up"
            }
          ]
        }
      ]
    },
    {
      "description": "right_alt + g = page down",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_alt"
              ]
            },
            "key_code": "g"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "page_down"
            }
          ]
        }
      ]
    },
    {
      "description": "right_control + e = up",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_control"
              ]
            },
            "key_code": "e"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "up_arrow"
            }
          ]
        }
      ]
    },
    {
      "description": "right_control + d = down",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_control"
              ]
            },
            "key_code": "d"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "down_arrow"
            }
          ]
        }
      ]
    },
    {
      "description": "right_control + f = right",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_control"
              ]
            },
            "key_code": "f"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "right_arrow"
            }
          ]
        }
      ]
    },
    {
      "description": "right_control + s = left",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_control"
              ]
            },
            "key_code": "s"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "left_arrow"
            }
          ]
        }
      ]
    },
    {
      "description": "right_control + t = page up",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_control"
              ]
            },
            "key_code": "t"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "page_up"
            }
          ]
        }
      ]
    },
    {
      "description": "right_control + g = page down",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "modifiers": {
              "mandatory": [
                "right_control"
              ]
            },
            "key_code": "g"
          },
          "to": [
            {
              "repeat": true,
              "key_code": "page_down"
            }
          ]
        }
      ]
    }
  ]
}

To enable these mappings on MacOS (assuming that you have already installed Karabiner):

1. Copy the JSON in this gist
2. Open https://genesy.github.io/karabiner-complex-rules-generator/
3. Paste the JSON (see screenshot)
4. Click the install button, which will open Karabiner
5. Enable these new rules