Added note about ticket numbers on macro upgrade instructions (#354)

Author: mo-lucy-gordon

source/WorkingPractices/macros.rst

@@ -43,6 +43,11 @@ Within the file a blank upgrade macro will typically look like this:
           # Input your macro commands here
           return config, self.reports
 
+Note: The BEFORE_TAG should match the AFTER_TAG of the previous macro in the chain. So if this is not the first macro since the release then the BEFORE_TAG will be the version number with an added ticket number as well. For example:
+
+.. code-block::
+
+      BEFORE_TAG = "vn13.0_t123"
 
 Example of an upgrade macro
 ---------------------------
@@ -132,4 +137,4 @@ The ``vnXX.Y_tTTTT`` option must match the After Tag of your upgrade macro. When
 
 .. tip::
 
-    The apply_macros script requires python >= 3.9. At the Met Office this can be achieved by ``module load scitools``.
+    The apply_macros script requires python >= 3.9. At the Met Office this can be achieved by ``module load scitools``.