how do I get these maps on my xbox?

You need to add the forger as a friend, then locate them on your friends list within Halo 5 and press A. Go to Map Variants. Find what you're looking for under either 'Bookmarks' or 'Files'. Push in on the left thumbstick to download.
It will then show in your Bookmarks, and be available in custom games and forge.

 

From your Xbox One:

1. Open your guide
2. Navigate to "Friends"
   
3. Select "Find Someone"
4. Enter the Gamertag of the player you want to download the map from.
5. Select "Add Friend"
6. Open Halo 5: Guardians (if it isn't open already)
7. Hit 'X' for "Friends"
8. Use the Right Bumper to navigate to "Friends"
   
9. Select the player your just added. Note: it may take a few minutes for a newly added friend to appear in your Halo 5: Guardians Friends List.
   
10. Navigate to File Browser section at the bottom of their profile and select either "Map Variants" or "Game Mode Variants."
    
11. Find the name of the file you were looking for either under Bookmarks or Files.
12. Click Left Stick to "Add Bookmark."
13. In a Custom Games lobby the map or game mode you bookmarked will appear under "Bookmarks" when you select to change the map or game mode.

 

Thank god we no longer have to follow the direct link to Bungie's file share, log in, and click download anymore. Those were tough times back in 2007. As @ToRn1ne states, it's just a simple 13 steps now. **Pheh** Crisis averted.

As an added bonus the privacy you used to have has now been completely circumvented and every file you have stored is shared to the world. Thank you 343 for helping to make a much more open and advanced file share system.

 

The sarcasm is strong in this one.