YoYoK3nny's instructions were great, but some people have had trouble finding the Game ID and Upload ID so I'll try to give some more details.
To find both ID's you need to inspect the source code of the game's webpage. Game ID is easy, Upload ID can be harder.
Instructions for finding Game ID (in Firefox):
- Right-click anywhere on the game's webpage (except for an image), and select -> Inspect (Q). The Devtools menu (F12) will come up with the Inspector already selected
- On the far lefthand side you'll see a 'Search HTML' box next to a magnifying glass icon. Enter your first seach term here, and hit enter:
itch:path - There should only be one entry, it will be something like the below. The number is the Game ID you need for the JSON file.
<meta name="itch:path" content="games/123456">
Instructions for finding Upload ID (in Firefox):
- Right-click anywhere on the game's webpage (except for an image), and select -> Inspect (Q). The Devtools menu (F12) will come up with the Inspector already selected
- On the far lefthand side you'll see a 'Search HTML' box next to a magnifying glass icon. Enter your first seach term here, and hit enter:
data-upload_id - There will likely be multiple entries, correlating to the download links available on the page for the game. If you click on the first entry, it will highlight in the page code the exact download link it's referring to.
For example, if your game has a download for MAC and a download for PC (in that order), and you want the PC download, then click on the second "data-upload_id" entry in the Inspector to highlight it in the webpage - it should have highlighted the Download link for the PC download. Use that code as your Upload ID in the JSON file, example:
<a data-upload_id="4567890" href="javascript:void(0);" class="button download_btn">Download</a>
Instructions for Chrome are basically the same, you still right-click -> Inspect, the F12 Developer tools page will come up -> click on the Search tool tab (at the bottom, towards in the middle), and find your variable eg 'itch:path' or 'data-upload_id', if it cannot find 'data-upload_id' try 'data-upload' because Chrome's search was not as reliable when I tried it. It also doesn't highlight the download button for the specific download in the live webpage (like Firefox does), so instead you'll need to read the code next to the highlighted upload_id snippet, to try to see which download it's referencin. Eg if you're searching for the PC download from Mega, something like this will be in the code below the upload_id reference:
<strong class="name" title="💻 GAME NAME - PC (MEGA)">💻 GAME NAME - [1.0] - PC (MEGA)</strong>