I think this is what you're asking for, if not, feel free to send me a PM and elaborate on what you need help with and I'll be happy to help.

I take it you'll have already configured your USB drive with your Xbox? If not plug it into your xbox, go to dashboard, and storage, you should see it, click on it and it should prompt you to configure it, when it configures it's good to go. Just a quick note, when you configure a USB it wipes everything off of it, so make sure it's empty beforehand.

The next step is to make a copy of a gamesave already on your xbox and put it on the USB, you'll see why later.

Next, seeing as you already have horizon, plug the usb into your PC and open up horizon. On the right hand side you should see the device explorer and it should automatically open up your USB device.

Now drag the gamesave already on your PC into horizon, if it still doesn't open, open the package manager tool in horizon and use the "open" feature to manually open the save file in Horizon. After you've done that, open up the save you put on your USB by using the device explorer, once opened you'll have two saves open, one from your PC and one from your Xbox. You should see Console ID, Device Id and profile ID, delete the entries for these on the save from your PC and copy and paste the one's from the save from your Xbox into the fields onto the save from your PC, once done, save rehash and resign the save from your PC and close both saves. Then go into the device explorer click inject new file and select the save from your PC, you should get a message if it transferred successfully. Once done, you can unplug your USB and put it back into your Xbox, and the save should be ready to use.

That should be it, if not, feel free to hit me up with any other questions. Here's also a video tutorial to aid you.