how can i customise workflow
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.
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,
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