how can i customise workflow
This is really a vague description.
You can checkout the official documentation here:
https://confluence.atlassian.com/adminjiraserver072/working-with-workflows-828787890.html
It's well detailed for basic usage,
This is a very basic function of an admin. I would reference the documentation as a start. The link I provided is for cloud so if you are on server then you should search the server document. Note you will need to be an admin to do this.
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