How to generate texture coordinates for a rectangle?

I've got a function which generates geometry data for a rectangle. So far I've got the vertex, indices and the normals nailed, but I have no idea how to generate the texture coordinates - so far I've only used textures with imported 3d models, but now I want to generate my own rectangle on the fly and so need to generate the texcoords aswell.

So how do I make the texcoords? I am not sure how the mapping works.

bool CreateRectangleData(const float sizeX, const float sizeY, const float sizeZ, std::vector<float>& vertexData, std::vector<float>& normalData, std::vector<float>& texcoordData, std::vector<uint32_t>& indiceData)
{
    if (sizeX <= 0 || sizeY <= 0 || sizeZ <= 0)
        return false;

    const float X = sizeX / 2.0f;
    const float Y = sizeY / 2.0f;
    const float Z = sizeZ / 2.0f;

    float cubeVertices[] = {-X, -Y, -Z, -X, -Y, Z, X, -Y, Z,
                            X, -Y, -Z, -X, Y, -Z, -X, Y, Z,
                            X, Y, Z, X, Y, -Z, -X, -Y, -Z,
                            -X, Y, -Z, X, Y, -Z, X, -Y, -Z,
                            -X, -Y, Z, -X, Y, Z, X, Y, Z,
                            X, -Y, Z, -X, -Y, -Z, -X, -Y, Z,
                            -X, Y, Z, -X, Y, -Z, X, -Y, -Z,
                            X, -Y, Z, X, Y, Z, X, Y, -Z};

    float cubeNormals[] = {0.0f, -1.0f, 0.0f, 0.0f, -1.0f, 0.0f, 0.0f, -1.0f, 0.0f,
                           0.0f, -1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f,
                           0.0f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 0.0f, -1.0f,
                           0.0f, 0.0f, -1.0f, 0.0f, 0.0f, -1.0f, 0.0f, 0.0f, -1.0f,
                           0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f,
                           0.0f, 0.0f, 1.0f, -1.0f, 0.0f, 0.0f, -1.0f, 0.0f, 0.0f,
                           -1.0f, 0.0f, 0.0f, -1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f,
                           1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f, 1.0f, 0.0f, 0.0f};

    float cubeTexCoords[] = {};  // ???

    uint32_t cubeIndices[] = {0, 2, 1, 0, 3, 2, 4, 5, 6,
                              4, 6, 7, 8, 9, 10, 8, 10, 11,
                              12, 15, 14, 12, 14, 13, 16, 17, 18,
                              16, 18, 19, 20, 23, 22, 20, 22, 21};

    vertexData.insert(vertexData.begin(), cubeVertices, cubeVertices + (sizeof(cubeVertices) / sizeof(float)));
    normalData.insert(normalData.begin(), cubeNormals, cubeNormals + (sizeof(cubeNormals) / sizeof(float)));
    texcoordData.insert(texcoordData.begin(), cubeTexCoords, cubeTexCoords + (sizeof(cubeTexCoords) / sizeof(float)));
    indiceData.insert(indiceData.begin(), cubeIndices, cubeIndices + (sizeof(cubeIndices) / sizeof(uint32_t)));

    return true;
}

Thanks