Add hint for changed port

HPC_Python_Guide.md

@@ -135,8 +135,9 @@ You should now be able to install packages with pip install <PACKAGE> as normal.
 
 1. Open a terminal on the cluster, either through ThinLinc or ssh.
 
-2. Make sure you are on a real node as described in section I.
-   If you are using ssh, it is good to start a tmux session by running:
+2. Call `sxm2sh` or `linuxsh`, as described in section I, so you are not on a login node.
+
+3. Start a tmux session by running:
    ```
    tmux
    ```
@@ -147,14 +148,14 @@ You should now be able to install packages with pip install <PACKAGE> as normal.
    > ```
    > in a terminal on the cluster.
 
-3. Take note of the node's hostname. You can see this by running:
+4. Take note of the node's hostname - you will need it later. You can see this by running:
    ```
    echo $HOSTNAME
    ```
 
-4. Navigate to your project folder and activate the virtualenv.
+5. Navigate to your project folder and activate the virtualenv.
 
-5. Start a Jupyter lab or Jupyter notebook server by entering one of the following:
+6. Start a Jupyter lab or Jupyter notebook server by entering one of the following:
    ```
    jupyter lab --port=44000 --ip=$HOSTNAME --no-browser
    jupyter notebook --port=44000 --ip=$HOSTNAME --no-browser
@@ -168,21 +169,23 @@ You should now be able to install packages with pip install <PACKAGE> as normal.
         or http://127.0.0.1:44000/lab?token=401720c25a3e9411a5f28d9015591b19a9032fc90989ffa0
    ```
 
-6. Open a terminal on your own computer and run
+7. Open a terminal on your own computer and run
    ```
    ssh <USERNAME>@login2.hpc.dtu.dk -g -L44000:<HOSTNAME>:44000 -N
    ```
-   where `<USERNAME>` is your DTU user name and `<HOSTNAME>` is the hostname you found in step 3.
+   where `<USERNAME>` is your DTU user name and `<HOSTNAME>` is the hostname you found in step 4.
    This should prompt you for your DTU password, _**and then NOTHING SHOULD HAPPEN**_.
 
-7. Open your browser and enter the URL printed in step 5 that starts with `127.0.0.1`
+8. Open your browser and enter the URL printed in step 5 that starts with `127.0.0.1`
    (e.g. `http://127.0.0.1:44000/lab?token=401720c25a3e9411a5f28d9015591b19a9032fc90989ffa0`).
    This should open the Jupyter interface. Any commands you run will be executed on the HPC
    cluster. 
    
-   > **NOTE:** If no URL beginning with `127.0.0.1` was printed in step 5, change the first part
+   > **Troubleshooting:** If no URL beginning with `127.0.0.1` was printed in step 5, change the first part
    > manually to `127.0.0.1` before entering it in your browser. In the example from step 5, you
-   > would change `n-62-20-9` to `12.0.0.1`.
+   > would change `n-62-20-9` to `127.0.0.1`.
+
+   > **Troubleshooting:** If the number after `htttp://127.0.0.1:` is not `44000`, Jupyter selected another port. In this case, redo step 7 where 44000 is replaced with the number from the URL printed by Jupyter. This happens if the port we request with `--port=44000` is not available.
 
 If you close your browser, you can reconnnect by entering the URL again.
 If you lose your internet connection, you can reconnect by repeating steps 5 and 6.