Having trouble implementing ACTION_OPEN_DOCUMENT to my project

In onActivityResult(), call takePersistableUriPermission() on a ContentResolver, passing in the Uri that you got along with the mode flag(s) that indicate what access you want (read, write, both).

To handle permission results overide onRequestPermissionsResult as below

 @Override
 public void onRequestPermissionsResult (int requestCode, String[] permissions, int[] grantResults) {
    super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    switch (requestCode) {
        case General.REQUESTPERMISSION:
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                //reload my activity with permission granted or use the features that required the permission

            } else {
                Messenger.makeToast(getContext(), R.string.noPermissionMarshmallow);
            }
            break;
    }
}

and to persist the permission implement as shown below in your onActivityResult method

@Override
public void onActivityResult (int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    //if ok user selected a file
    if (resultCode == RESULT_OK) {
        Uri sourceTreeUri = data.getData();
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
            getContext().getContentResolver().takePersistableUriPermission(sourceTreeUri, Intent.FLAG_GRANT_READ_URI_PERMISSION | Intent.FLAG_GRANT_WRITE_URI_PERMISSION);
        }
    }
}