Create (function, procedure) and trigger to increment a counter

I have a table usage, I would like to create a (function or procedure) called by the trigger to increment column counter after an update.

Can someone lend me a hand with the process behind creating this function,procedure and trigger.

Table "public.usage"
Column | Type | Modifiers
---------------+-----------------------+-----------
instrument | character varying(13) |
date | date |
counter | integer |

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