Updating.md

@@ -129,6 +129,8 @@ In addition, you must [create a new public key pair](https://gitlab.torproject.o
 
 Testing the `nightly` channel is easiest. The URL can be modified in the [build](https://gitlab.torproject.org/tpo/applications/tor-browser-build/-/blob/main/projects/firefox/build#L105) script. If you are testing `release` or `alpha` then a more complicated patch will be needed.
 
+The URL you are setting will contain the update responses, that are some XML files that the Firefox updater polls with a certain frequency, to check if any update is available.
+
 ## Signature Public Certificate
 
 After creating the new testing key pair, add the new certificate into tor-browser by overwriting the appropriate secondary key. For `nightly`, you can achieve this with a patch
@@ -206,3 +208,65 @@ mkdir signed
 
 $mar_tools/signmar -d $nssdb -n marsigner -v signed/$marfile
 ```
+
+## Prepare the update responses
+
+As written above, we need to create the XML files to tell Firefox where it can find our mar files.
+
+In production, we use a few tools in `tor-browser-build` to create the update responses, let's use them also for our test!
+
+They are located in `tools/update-responses`.
+
+### Create/update the `config.yml`
+
+`config.yml` is a non-versioned file that is generated automatically at a certain point during the usual builds, so you might have already one, and you can adapt it to your test needs.
+If you do not have one already, you can copy `tools/signing/nightly/update-responses-base-config.yml` to it and start from there.
+
+You should update at least the `mars_url` property and replace occurrences of `nightly_version` with the actual version you have built. You might need to update `platformVersion`, too.
+
+For example (only the changes, not the full file):
+
+```yaml
+download:
+    mars_url: https://tb-build-05.torproject.org/~pierov/torbrowser/nightly
+channels:
+    nightly: tbb-nightly.2023.03.06
+versions:
+    tbb-nightly.2023.03.06:
+        platformVersion: 102.8.0
+        incremental_from:
+          - tbb-nightly.2022.03.05
+```
+
+`~/public_html/torbrowser` in this case is a link to the `torbrowser` directory in the clone of `tor-browser-build` when I ran the build.
+
+If you have used a build server, but are creating the update responses locally, you can use the `releases_dir` property to tell the scripts where they can find your builds.
+It must contain at least the `.mar` archives and the `mar-tools`.
+
+Finally, if you want to generate incrementals, you need to create a `versions/$yourVersion/incremental_from` array, with the version(s) you want to generate incrementals from.
+
+### Generate the incrementals (optional)
+
+Normally, we would generate the incrementals with some `make` command.
+
+However, we do not have a recipe for that in the `Makefile`, so you will need to run this command:
+
+```sh
+NO_CODESIGNATURE=1 ./gen_incrementals nightly
+```
+
+`NO_CODESIGNATURE=1` is needed only on macOS (unless you signed your nightly build for some reason).
+
+The resulting incremental mars are not signed (maybe we could add a `config.yml` entry to specify the nssdb path and the nickname of the key to use), so you will need to sign them with the instructions you can find above.
+
+### Generate the update responses
+
+Finally, creating the update responses is very easy, just run this command:
+
+```sh
+./update_responses
+```
+
+When it is done, copy the content of the `htdocs` directory to the URL you provided to the build (e.g., `~/public_html/update_3`).
+
+At this point, Firefox should find the update automatically, download it, and apply it.
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