Xbox Custom Gamerpicture and Gamerscore Modding
Prerequisites:
Windows 10
Xbox App [ Register or Signin to view external links. ]
Avatars App [ Register or Signin to view external links. ]
Fiddler [ Register or Signin to view external links. ]

Tutorial Setup:
1) Install Fiddler and open Tools>Telerik Fiddler Options
2) Go to the HTTPS tab, click the "Actions" button, Trust Root Certificate and go through the Yes dialog windows until the cert is in your trusted root
3)Tick the "Decrypt HTTPS Traffic" box, then for the "Protocols" section, paste in:



4) Click Ok, and restart Fiddler.
5) Click the "WinConfig" button, and look for "Xbox" & Xbox Avatars" Tick the boxes, save changes. Or, if you're lazy, click "Exempt All" then save changes. This allows you to record local loopback traffic, bypassing the security mechanism implemented by MS w/ Win8
6) Make sure you're recording (File>Capture Traffic) and we can begin


Gamerpic Tutorial Procedure:
1) Open the Xbox Avatars App, you should start to see some traffic in Fiddlers left hand panel
2) Click the camera icon on the left-hand side, then click the camera on the right-hand side (We're setting a gamerpic of our avatar to grab the link for gamerpics)
3)Click "Save as Gamerpic" [ Register or Signin to view external links. ]
4) In Fiddler, on the left-hand side, look for: [ Register or Signin to view external links. ]
5) Copy the entirety of that request, and bring it over to the "Composer" tab
6) Mimic yours to look like this: [ Register or Signin to view external links. ]
7) Set the value to 0, then click "Execute" This should return as "201 Created" (We do this to have our gamerpic show instead of our avatar)
8) Next, look for the gamerpic link itself: [ Register or Signin to view external links. ]
9) Same as before, copy that request and throw it in the composer tab
10) This is where Fiddler comes in handy: The upload file button - Have any image you'd like ready, .jpg or .png (Has to be 1080x1080), and pick your desired picture [ Register or Signin to view external links. ]
11) Should look like this: [ Register or Signin to view external links. ]
12) Click the "Execute" button and your custom gamerpic will be set!
13) Example of completion: [ Register or Signin to view external links. ]


Gamerscore Tutorial Procedure:
1) Start recording in Fiddler, and open the Xbox App
2) Change a simple setting, such as your profile bio
3) Grab that link and throw it in the composer
4) Change the Bio setting to Gamerscore, and enter any value you desire (Max is 999,999,999)
5) Your composer should look like this: [ Register or Signin to view external links. ]
6) Click "Execute" and you'll have your new gamerscore
7) Example of Completion: [ Register or Signin to view external links. ]



Edit: Rewriting right now.